T« 


PLATE   I. 

U.  S.  Naval  Radio  Station,  San  Diego,  California.     View  of  Transmitting 
Station,  Chollas  Heights,  operated  by  distant  control.     (See  Par.  177.) 


Frontispiece. 


ELEMENTS 


OF 


RADIOTELEGRAPHY 


BY 


ELLERY  W.  STONE 

LIEUTENANT,  U.S.N.R.F. 

Member  Institute  of  Radio  Engineers 

U.  S.  Naval  Institute 


725  ILLUSTRATIONS— 33  PLATES 


NEW  YORK 

D.   VAN    NOSTRAND    COMPANY 
25  PARK  PLACE 

1919 


Copyright,  1919 
D.  VAN  NOSTRAND  COMPANY 


PUBLISHED  BY  PERMISSION  OF  THE 
U.  S.  NAVY  DEPARTMENT 


PRESS  OF 

THE  NEW  ERA  PRINTING  COMPANY 
LANCASTER,  PA. 


FOREWORD. 

Although  this  text  was  written  primarily  for  the  guidance 
and  instruction  of  radio  students  hi  the  Communication 
Service  of  the  Navy,  it  is  believed  that  it  will  be  found  help- 
ful hi  radio  instruction  hi  the  other  military  branches  of 
the  Government,  hi  civilian  radio  schools,  and  for  the  self 
instruction  of  those  interested  in  the  subject. 

Every  attempt  has  been  made  to  present  the  subject 
from  the  physical  rather  than  the  mathematical  standpoint 
without  sacrifice  of  technical  accuracy,  in  order  that  the 
subject  matter  may  be  readily  grasped  by  the  layman.  In 
the  study  of  the  text,  a  knowledge  of  elementary  physics 
and  simple  mathematics  is  desirable  but  not  necessary. 

It  is  believed  that  the  logical  instruction  of  the  student 
should  follow  the  development  of  the  art  by  its  inventors 
and  scientific  workers,  consequently  considerable  promi- 
nence is  given  to  the  patent  phase  of  the  subject. 

The  text,  in  the  main,  is  a  resume  of  a  series  of  lectures 
given  to  the  radio  classes  at  this  station,  and  was  put  into 
written  form  at  the  request  of  the  men  under  instruction. 

Acknowledgment  is  gratefully  made  to  the  authors  of 
the  publications  noted  in  the  bibliography,  to  Chief  Elec- 
trician (RO)  R.  B.  Black,  U.S.N.R.F.,  for  the  diagrams 
appearing  in  the  text,  to  Ensign  Rudolph  Oeser,  U.S.N., 
for  assistance  in  its  preparation,  to  Dr.  Erich  Hausmann 
of  the  Polytechnic  Institute  of  Brooklyn  for  many  valuable 
suggestions,  and  to  the  following  for  then-  courtesy  in  sup- 
plying photographs:  Dr.  F.  A.  Kolster  of  the  Bureau  of 
Standards;  Federal  Telegraph  Co.,  Haller-Cunningham 
Electric  Co.,  and  the  Moorhead  Laboratories,  of  San 
Francisco;  Kilbourne  &  Clark  Manufacturing  Co.  of 
Seattle;  Weston  Electrical  Instrument  Co.  of  Newark, 
and  General  Radio  Co.  of  Cambridge. 

ELLERY  W.   STONE. 

U.  S.  Naval  Radio  Station, 
San  Diego,  California, 
March,  1919. 

iii 

M38073 


TABLE   OF   CONTENTS. 

CHAPTER   ONE. 

Page 

I.     Principles  of  Radiotelegraphy 1 

II.     Electrical  Terms 6 

III.  Condensers 10 

IV.  Inductances 13 

V.     Electro-Magnetic  Induction 15 

VI.     Alternating  Current 16 

CHAPTER   TWO. 

VII.     Damping  and  Resonance 27 

VIII.     Logarithmic  Decrement 30 

IX.     Wave  Length,  Frequency,  Time  Period ...  35 

CHAPTER  THREE. 

X.     The  Marconi  1896  Transmitter 41 

XI.     Coupled  Circuits 44 

XII.     Lodge  1898  Transmitter 51 

CHAPTER  FOUR. 

XIII.  Theory  of  lonization 57 

XIV.  Spark  Gaps 59 

XV.     Marconi  1900  Transmitter 66 

XVI.     The  Quenched  Spark  Gap 69 

XVII.     The  Telefunken  Transmitter 70 

CHAPTER  FIVE. 

XVIII.     The  Four  Radio  Transmitter  Circuits 74 

XIX.     Transmitting  Keys 75 

v 


TABLE   OF   CONTENTS. 

Page 

XX.  Transformers 79 

XXI.  Condensers 88 

XXII.  Modern  Spark  Gaps 92 

XXIII.  Transmitting  Inductances 94 

XXIV.  Antenna  Current  Ammeter 98 

XXV.  Antenna  Condenser 101 

XXVI.     Antenna  Switch 103 

CHAPTER  SIX. 

XXVII.  Complete  Transmitter 106 

XXVIII.  Marconi  System 110 

XXIX.  Telefunken  System Ill 

XXX.  Kilbourne  &  Clark  System Ill 

XXXI.  Haller  Cunningham  System 120 

XXXII.  Fessenden  System 121 

XXXIII.  Multitone  System 122 

XXXIV.  French  Postal  and  Telegraph  Department 

System 124 

CHAPTER   SEVEN. 

XXXV.     Wave  Meters 127 

XXXVI.     Decremeters 132 

XXXVII.     Adjustment  of  a  Modern  Transmitter 139 

CHAPTER   EIGHT. 

XXXVIII.     Undamped  Wave  Transmitters 146 

XXXIX.     The  Poulsen  Arc  Transmitter 149 

XL.     Poulsen  Arc  Keys 164 

CHAPTER   NINE. 

XLI.     Antennae 169 

XLII.     Various  Types  of  Antennae 175 

XLIII.     Tower  Construction 181 

vi 


TABLE   OF  CONTENTS. 

Page 

XLIV.     Earth  Connections 182 

XLV.     Antenna  Resistance 187 

XLVI.     Wave  Propagation 193 

XL VII.     Aerial  Communication 200 

CHAPTER  TEN. 

XLVIII.     Pioneer  Receivers 203 

XLIX.     Detectors 209 

L.     Modern  Receivers 222 

LI.     Receiving  Transformers 224 

LII.     Receiving  Condensers 227 

LIII.     Telephone  Receivers 229 

LIV.     Audibility  Measurements 233 

LV.     Harmonic  Oscillation  of  Receivers 234 

CHAPTER  ELEVEN. 

LVI.     The  Edison  Effect 236 

LVII.     Electron  Tube  Detectors 239 

LVni.     Electron  Tube  Amplifiers 247 

LIX.     The  Heterodyne 248 

LX.     Audion  Beat  Receiver 251 

LXI.     Modern  Electron  Tubes 252 

LXII.     Magnetic  Control 254 

LXIII.     Conclusion 254 

APPENDIX. 

Bibliography 256 

Index                                                              .  259 


vn 


CHAPTER   ONE. 

I.  •        -     -'-'U^ 

PRINCIPLES   OF  RADIOTELEGRAPHY. 

1.  If  a  piano  string  be  struck,  or  a  violin  plucked  or 
bowed,  so  as  to  set  it  into  vibration,  the  mechanical  vibra- 
tion of  the  string  will  cause  the  emission  of  a  sound  wave 
which  travels  out  from  the  vibrating  string  in  all  direc- 
tions.   A   mechanically  vibrating  body  thus  radiates  a 
wave ;  in  this  case  an  air  wave.    As  a  matter  of  fact,  in  the 
case  of  a  piano  and  of  a  violin,  the  vibrating  string  is  em- 
ployed to  vibrate  a  sounding  board  which  in  turn  radiates 
the  wave  heard.     In  violin  construction,  great  care  must 
be  taken  to  see  that  the  body  of  the  violin  will  respond 
equally  well  to  all  of  the  notes  which  may  be  played  on  it. 
In  other  words,  the  body  must  have  no  pitch  or  period  of 
its  own — it  must  be  non-periodic,  or  aperiodic  as  it  is 
termed. 

2.  If  an  iron  poker  be  heated  in  a  flame,  before  it  has 
actually  begun  to  turn  red  from  the  heat,  the  radiation  of 
a  heat  wave  from  it  may  be  observed  by  holding  it  near 
the  hand  or  face.    As  it  is  further  heated,  it  turns  a  dull, 
cherry  red,  then  "red  hot"  and  finally  "white  hot."     In 
heating  the  poker,  we  have  set  the  molecules  of  iron  of 
which  it  is  formed  into  vibration.    The  more  we  heat  the 
poker,  the  faster  the  molecules  vibrate ;  or  we  may  say,  the 
faster  they  vibrate,  the  hotter  the  poker  becomes.     Just  as 
in  the  case  of  the  vibrating  piano  string,  so  these  vibrating 
molecules  radiate  a  wave,  only  in  this  instance,  instead  of 
radiating  a  sound  wave,  they  emit  a  heat  wave. 

I 


3]          ELEMENTS  OF  RADIOTELEGRAPHY. 

3.  These  waves  are  exactly  similar  to  waves  on  water. 
They  are  composed  of  troughs  and  crests.    The  distance 
between  one  crest  and  the  next  succeeding  one  is  termed 
the  wave  length.    The  faster  the  string  or  the  iron  mole- 
cules vibrate,  the  more  waves  they  will  radiate  in  a  given 
length  of  time.    The  faster  the  waves  are  radiated,  the 
more  closely  they  will  follow  or  succeed  each  other.    The 
number  of  waves  radiated  per  second  is  called  the  /re- 
quency.    The  faster  the  vibration,  the  greater  the  number 
of  waves  radiated,  and  thus  the  greater  the  frequency. 
But  since,  with  the  greater  frequency,  the  waves  succeed 
each  other  more  closely,  the  less  will  be  the  distance  be- 
tween any  two  successive  waves  and  hence  the  shorter 
the 'wave  length.    Thus,  the  frequency  of  the  wave  varies 
inversely  as  the  wave  length,  or  vice  versa.    If  we  double 
the  frequency,  the  wave  length  is  cut  in  half ;  if  we  increase 
the  wave  length  three  times,  the  frequency  is  cut  to  one 
third. 

4.  We  have  seen  that  a  mechanically  vibrating  body  will 
radiate  sound  waves  and  that  they  are  detected  thru  the 
medium  of  the  ear — thru  the  medium  of  the  sense  of 
sound.    But  there  are  some  sound  waves  which  the  ear 
cannot  detect.    If  a  sound  wave  occurs  at  a  frequency  be- 
low 16  per  second,  that  is  to  say,  16  waves  sent  out  from 
the  string  in  one  second  of  time,  the  ear  will  not  detect  it 
—  it  will  be  too  low  a  pitch.     On  the  other  hand,  if  the  note 
pitch  or  frequency  be  over  30,000  per  second,  the  average 
ear  will  not  be  able  to  hear  it.     So  we  say  that  the  limits  of 
audibility  are  between  16  and  30,000  per  second.    A  cricket 
can  chirp  so  shrill,  so  high,  a  note  that  some  people  cannot 
detect  it.     The  frequency  of  the  note  is  thus  beyond  their 
limit  of  audibility — they  cannot  "tune"  their  ears  to  so 
high  a  frequency,  so  short  a  wave  length. 

2 


PRINCIPLES   OF   RADIOTELEGRAPHY.         [6 

5.  In  the  poker  experiment,  the  color  of  the  metal  grad- 
ually changed  from  a  dull  red  to  a  dazzling,  incandescent 
yellow.     We  were  aware  of  the  radiation  from  the  heated 
metal  by  its  color  as  detected  by  the  sense  of  sight.     But 
before  the  poker  had  even  turned  red  there  was  a  radiation 
from  it  which  could  not  be  seen  yet  which  we  could  feel. 
It  is  obvious,  therefore,  that  just  as  the  ear  has  certain 
limitations  of  responsiveness,  so  the  eye  as  well  can  re- 
spond to  vibrations  of  but  certain  frequencies.    In  commenc- 
ing to  heat  the  poker,  the  energy  imparted  to  it  from  the 
flame,  and  which  set  the  molecules  into  vibration,  was  not 
very  great  so  that  they  vibrated  slowly.    The  waves  of 
heat  sent  out  did  not  follow  each  other  very  closely.     But 
as  more  and  more  heat  was  applied,  the  molecules  vibrated 
more  rapidly  and  the  wave  length  became,  as  we  have  seen 
in  paragraph  3,  shorter.     The  highest  rate  of  vibration  or 
oscillation  was  reached  when  the  poker  became  yellow  in 
color  and  the  wave  length  for  this  color  was  the  shortest. 
In  changing  color  from  cherry  red,  successively  to  bright 
red,  orange,  and  finally  to  yellow,  we  have  had  waves  sent 
out  from  the   heated   metal,   differing   from  each  other 
only  in  that  their  length  became   shorter  and   shorter. 
Color — then — as  we  detect  it  by  the  eye,  is  merely  a  dis- 
tinction between  light  waves  of  different  length.     Waves 
striking  the  retina  of  the  eye  at  different  rates  of  vibration 
cause   different  vibrations   or  impressions   on  the  retina 
to  be  recorded,  and  these  different  impressions  we  call 
color. 

6.  The  color  red,  we  have  seen,  has  a  longer  wave 
length  than  the  color  yellow.    The  colors  of  the  rainbow, 
red,  orange,  yellow,  green,  blue  and  violet  range  succes- 
sively in  wave  length  from  about  750  millionths  of  a  milli- 
meter (0.00075  mm.)  to  about  380  millionths  of  a  milli- 

3 


7]          ELEMENTS  OF  RADIOTELEGRAPHY. 

meter  (0.00038  mm.).     The  deepest  shade  of  red  has  thus 
about  twice  the  wave  length  of  the  lightest  shade  of  violet. 

7.  We  observed,  however,  that  there  was  a  heat  radia- 
tion from  the  hot  poker  which  we  could  not  see.     Since 
the  poker  at  this  time  had  not  turned  red,  the  wave  length 
must  have  been  longer  than  that  of  red.     In  other  words, 
its  wave  length  was  over  0.00075  mm.     Such  waves  are 
called  infra-red  waves,  "below  red"  waves.     Similarly, 
above  the  upper  end  of  the  spectrum,  which  is  the  range  of 
visible  light  waves  in  the  order  of  color  given  in  paragraph 
6,  there  are  radiations  of  a  wave  length  shorter  than  that 
of  violet  and  which,  like  the  infra-red  waves,  are  invisible 
to  the  eye.    These  waves  are  called  ultra-violet,  "  beyond 
violet,"  waves  or  rays.    These  rays  are  given  off  by  the 
sun,  the  electric  arc,  radium,  and  the  Crookes  or  "  X-Ray  " 
tube.     They  are  the  shortest  waves  of  which  we  have  cog- 
nizance. 

8.  A  sound  wave  must  have  some  tangible  medium  by 
which  it  can  be  conducted  or  propagated.     It  must  travel 
on  a  solid,  a  liquid  or  a  gas.     A  sound  wave  will  have  a 
different  velocity  of  propagation  depending  on  which  of  the 
three  mediums  over  which  it  is  sent.     A  sound  wave  can- 
not be  sent  thru  a  vacuum.     If  an  electric  bell  were  placed 
in  a  glass  jar  and  caused  to  ring,  and  the  air  gradually 
pumped  out  of  the  jar,  the  sound  of  the  bell  would  gradually 
die  away  as  the  air  was  removed,  until  when  a  state  of 
vacuum  was  reached  it  would  be  impossible  to  hear  it. 

9.  Light  waves,  however,  do  not  travel  on  such  a  me- 
dium.    A  light  wave  may  be  sent  with  perfect  ease  thru  a 
vacuum,  as  we  can  see  in  the  case  of  the  ordinary  electric 
lamp,  the  filament  of  which  is  enclosed  in  a  vacuum..     The 
medium  on  which  light  travels  is  called  the  luminiferous 

4 


PRINCIPLES    OF   RADIOTELEGRAPHY.       [12 

ether,  or  simply  the  ether.    It  is  a  medium  which  exists 
everywhere,  yet  of  which  we  have  very  little  knowledge. 

10.  We  have  seen  in  paragraph  7  that  the  only  difference 
between  heat,  light,  and  ultra-violet  waves  is  one  of  length. 
As  a  matter  of  fact,  they  are  all  electrical  waves  of  different 
length,  and  travel  thru  the  ether  at  a  rate  of  300,000,000 
meters  (186,000  miles)  per  second.     This  theory  was  pro- 
posed and  developed  in  1865,  by  J.  C.  Maxwell,  an  English 
physicist,  and  is  commonly  accepted. 

11.  Below  the  heat  or  infra-red  waves,  there  are  addi- 
tional electro-magnetic  waves  in  which  we  are  especially 
interested,  the  waves  employed  in  radiotelegraphy.    They 
range  in  length  from  about  125  meters,  the  shortest  wave 
length  used  in  submarine  sets,  to  40,000  meters,  a  wave 
length  which  an  American  inventor,  Peter  Cooper-Hewitt, 
has  already  employed.     It  should  be  borne  in  mind,  then, 
that  radio,  heat,  infra-red,  light,  and  ultra-violet  waves  are 
all  electric  waves  travelling  thru  the  ether  at  a  rate  of 
186,000  miles  per  second  and  differing  from  each  other 
only  in  the  frequency  of  their  vibration  or  their  wave 
length.     The  waves  used  in  radiotelegraphy  are  widely 
separated;  their  wave  length  is  the  greatest.     The  ultra- 
violet waves  succeed  each  other  with  tremendous  rapidity 
— their  frequency  is  the  highest — their  wave  length  is  the 
shortest. 

12.  In  paragraphs  1  and  2,  we  saw  that  vibrating  bodies 
radiated  waves,  either  sound  or  heat  as  the  case  might  be. 
To  produce  a  radio  or  electrical  wave,  which  is  the  subject 
with  which  we  are  concerned,  we  must  set  up  an  electric- 
ally vibrating  circuit.    The  various  types  of  modern  radio 
transmitters  are  thus  different  methods  of  causing  the 
vibration  of  an  electrical  circuit  which  will  radiate  a  wave. 

5 


13]         ELEMENTS  OF  RADIOTELEGRAPHY. 

13.  What  is  this  circuit  called?     It  is  termed  the  antenna 
or  aerial  circuit.    It  is  formed  of  an  elevated  conductor, 
consisting  of  a  network  of  wires  arranged  in  a  variety  of 
forms,  and  the  earth — connected  together  thru  a  network 
of  apparatus  by  which  this  circuit  is  set  into  electrical  vi- 
bration.    The  antenna  circuit  thus  corresponds  to  the  piano 
or  violin  string,  and  by  setting  it  into  vibration  (only  in  this 
case,  an  electrical  instead  of  a  mechanical  vibration),  we 
cause  it  to  radiate  a  wave.    The  detection  of  this  wave  at 
a  receiving  station  just  as  a  light  wave  is  detected  by  the 
eye  or  a  sound  wave  by  the  ear,  constitutes  the  system  of 
radio  communication. 

14.  In  order  that  we  may  understand  fully  how  this  an- 
tenna circuit  is  set  into  electrical  vibration,  it  will  be  neces- 
sary to  review  the  fundamental  principles  of  electricity. 

II. 
ELECTRICAL  TERMS. 

15.  Direct  current  is  an  electrical  current  flowing  in  but 
one   direction,   usually  non-pulsating.     Potential  is   that 
electromotive-force  (E.M.F.)  which  tends  to  drive  or  force 
a  current  of  electricity  thru  a  circuit.    It  corresponds  to 
the  head  or  pressure  which  produces  a  flow  of  water  in  a 
pipe.     Current  is  the  amount  of  electricity  flowing  thru  the 
circuit.     It  corresponds  to  the  flow  of  water  in  a  pipe.    In 
order  that  potential  may  force  current  thru  a  circuit,  it 
must  overcome  the  resistance  of  the  circuit.    The  resist- 
ance corresponds  to  the  friction  or  resistance  offered  to 
the  flow  of  a  stream  of  water  by  the  walls  of  a  pipe.    The 
resistance  of  an  electrical  circuit  depends  on  the  material 
of  which  the  conductor  is  made — silver  has  the  least  re- 
sistance and  copper  comes  second  on  the  list;  its  tempera- 

6 


ELECTRICAL    TERMS.  [17 

ture — with  most  conductors,  the  higher  the  temperature, 
the  greater  the  resistance;  its  length — the  resistance  of  a 
wire  is  directly  proportional  to  its  length;  and  the  area  of 
its  cross-section— its  resistance  is  inversely  proportional 
to  its  cross-sectional  area. 

16.  The  fundamental  law  of  electricity  is  known  as 
Ohm's  Law,  having  been  established  by  G.  S.  Ohm,  a 
German  physicist.     It  states  the  relationship  between  the 
potential,  the  current  and  the  resistance  of  a  circuit.    The 
potential  is  measured  in  volts.     (Named  after  Volta,  an 
Italian  physicist  of  the  early  nineteenth  century.)     The 
current  is  measured  in  amperes.     (Named  after  Ampere, 
a  French  physicist.)     The  resistance  is  measured  in  ohms. 

Potential  or  E.M.F.  is  denoted  by  the  letter  E. 
Current  is  denoted  by  the  letter  /. 
Resistance  is  denoted  by  the  letter  R. 
Ohm's  Law,  expressed  algebraically,  is 

/-f,  a) 

or  the  current  in  amperes,  /,  equals  the  potential  in  volts, 
E,  divided  by  the  resistance  in  ohms,  R.  Thus,  if  the  re- 
sistance of  a  circuit  is  10  ohms,  and  the  voltage  across  its 
terminals  is  120,  a  current  of  12  amperes  will  flow  in  the 
circuit. 

17.  The  rate  at  which  energy  is  expended  in  a  circuit 
per  second  is  measured  in  watts  (named  after  James  Watt 
who  made  the  earliest  researches  on  steam)  and  is  equal 
to  the  E.M.F.  multiplied  by  the  current.    It  is  thus  a  mea- 
surement of  power  and  is  represented  by  the  letter  P. 

From  the  above 

P  =  El.  (2) 

7 


18]         ELEMENTS   OF  RADIOTELEGRAPHY. 

18.  In  the  circuit  noted  in  paragraph  16,  the  power  equals 
the  potential  times  the  current,  which  equals   120  volts 
X  12  amperes  or  1,440  watts.     1,000  watts  equals  1  kilo- 
watt, so  1,440  watts  may  be  expressed  as  1.44  kw. 

19.  Since 

,_* 
'*' 

E  =  RI.  (3) 

Substituting  RI  for  £,  in  equation  (2) 

P  =  RII  =  I2R.  (4) 

So  that  power  in  watts  may  be  expressed  as  E.M.F.  times 
current  as  noted  in  paragraph  17  or  as  the  current  squared 

times  the  resistance  as  shown 

above' 


4  ohms  3ohms  20.  If  two  resistances  are 

Fig.  T.  connected  in  series  as  shown 

in  Fig.  1,  their  total  resistance 
will  be  equal  to  the  sum  of  their  separate  resistances,  or 

R  =  r,  +  r2.  (5) 

Thus,  the  total  resistance  of  the  connection  shown  in  Fig. 
1  is  4  ohms  plus  3  ohms  =  7  ohms. 

21.  The  reciprocal  of  resistance  is  called  conductance. 
It  is  represented  by  the  letter  G,  and  is  measured  in  mhos. 
Thus 

G-i.  (6) 

When  resistances  are  connected  in  parallel  as  shown  in 
Fig.  2,  their  total  resistance  is  given  by  the  formula 

8 


ELECTRICAL   TERMS.  [22 


R  =  -  (7) 


The  total  resistance  may  also  be  computed  from  the 
conductances,  for  from  equation  (7) 

G    =    01    +  02   +  03,  (8) 

and  R  may  be  obtained  from  equation  (6). 

Thus,  for  the  three  resistances  of  3,  4  and  5  ohms  shown 
in  Fig.  2,  the  conductance  is 


whence  the  resistance  is 

R  =  if  =  1.27  ohms. 

Thus,  when  two  resistances  are  connected  in  series, 
their  total  resistance  is  greater  than  either  of  them;  when 
they  are  placed  in  parallel,  the  total  is  less  than  either  of 
them.  If  all  the  resistances  are  of  the  same  resistance, 
for  example  3  ohms,  and  are  connected  as  shown  in  Fig. 
2,  their  total  resistance  is  the  resistance  of  one  of  them 
divided  by  their  number.  In  this  case,  the  total  resistance 
would  be  1  ohm. 

22.  A  substance  which  will  pass  electricity  thru  it  is 
termed  a  conductor.  One 

which  will  not  under  ordi-  *  A  A  A^A 

nary      circumstances,      is 
called  an  insulator  or  a  di- 
electric.    All    metals    and 
acidic    solutions    are    con- 
ductors, and  rarefied  gases  5 ohms 
will  pass  high  voltage  cur-                         Fig.  2. 
rents.    Glass,  hard  rubber, 

wood,  silk,  shellac,  compressed  air,  paper,  mica  and  many 
2  9 


23]         ELEMENTS   OF  RADIOTELEGRAPHY. 

other  substances  are  insulators  or  non-conductors  or  di- 
electrics. Wires  are  insulated  by  coating  them  with  any 
of  the  substances  noted  above,  and  when  so  insulated  will 
keep  the  current  within  the  wire  from  escaping  or  leaking 
to  some  other  conductor. 


« G/crss 


^-Copper 


III. 


CONDENSERS. 

23.  A  condenser  or  capacity  is  made  of  a  piece  of  dielec- 
tric coated  on  each  side  with  a  conductor.     A  familiar  type 

is  the  Ley  den  jar  shown  in  Fig.  3. 
(Devised  by  Musschenbroek  of  Ley- 
den  in  1746.)  By  connecting  both 
coatings  to  a  source  of  electricity, 
the  jar  may  be  charged.  The  larger 
the  charge  impressed  upon  a  Leyden 
jar  or  condenser,  the  greater  is  the 
E.M.F.  or  difference  of  potential 
between  the  two  metal  coatings  of 
the  condenser,  so  that  the  following  equation  holds  true, 

Q  =  EC  (9) 

where  Q  is  the  charge,  E  is  the  E.M.F.  or  difference  of 
potential,  and  C  is  the  capacity. 

24.  The  capacity  of  a  condenser  depends  on  that  area 
of  one  metal  coating  which  is  opposite  to  or  opposed  by 
the  other,  and  is  inversely  proportional  to  the  thickness  of 
the  dielectric  which  separates  them.     The  capacity  of  a 
condenser  is  given  by  the  formula 

AK 


Fig.  3- 


C  = 


47TG?' 


(10) 


where  C  is  the  capacity,  A  is  the  area  noted  above,  TT  is  the 

10 


CONDENSERS.  [25 

numeric  3.1416,  and  d  is  the  thickness  of  the  dielectric. 
K  is  a  constant  which  depends  on  the  particular  dielectric 
used  and  is  called  its  specific  inductive  capacity  or  dielec- 
tric constant.  Some  specific  inductive  capacities  are  given 
below : 

Hard  rubber .. 2.5        Paraffine 2.0        Air  (normal). 1.00059 

Glass 6.0-8.0         Turpentine. .  .  .2.2         Carbon  dioxide. . .  1.00090 

Mica 8.0        Petroleum ...  .3.1         Hydiogen 1.00028 

For  a  condenser  of  given  size,  then,  the  use  of  mica  as  a 
dielectric  would  give  the  condenser  four  times  the  capacity 
it  would  have  were  parafiine  used.  It  should  be  noted 
from  equation  (10),  that  the  thinner  the  dielectric,  the 
greater  the  capacity.  In  other  words,  if  a  condenser  were 
made  of  two  metal  plates  arranged  to  be  moved  with  re- 
spect to  each  other,  and  the  dielectric  were  air,  as  the  plates 
were  separated — the  capacity  would  be  reduced.  The 
greatest  capacity  would  be  obtained  by  separating  the 
plates  the  smallest  possible  distance  without  touching  them. 
This  principle  is  employed  in  the  design  of  variable  con- 
densers for  radio  receivers  (see  Fig.  114),  where  the  con- 
densers will  be  charged  only  to  very  low  potentials.  The 
capacity  of  a  condenser  is  measured  in  farads.  (After 
Michael  Faraday,  an  English  professor  of  the  nineteenth 
century.)  The  farad  is  too  large  a  unit,  however,  to  be  used 
practically,  so  that  capacities  are  rated  in  micro-farads,  i.e., 
millionths  of  a  farad.  The  capacity  of  a  condenser  may  be 
defined  as  that  property  of  a  condenser  by  virtue  of  which 
it  is  capable  of  storing  energy  hi  electrostatic  form. 

25.  If  condensers  are  connected  in  parallel,  their  total 
capacity  is  equal  to  the  sum  of  their  separate  capacities. 
This,  it  has  been  seen,  is  exactly  the  reverse  in  the  case 
of  resistances.  And  similarly,  the  same  formula  for  COR- 

11 


26]         ELEMENTS  OF  RADIOTELEGRAPHY. 

densers  in  series  as  for  resistances  in  parallel  applies. 
Thus 

C  =  ci  +  c2  +  c3  (11) 

for  condensers  in  parallel,  and  for  condensers  in  series, 

-r+ti- 

Ci        C2         C3 

Compare  with  equations  (5)  and  (7). 

26.  After  a  condenser  has  become  charged,  it  may  be 
discharged  by  connecting  the  two  metal  coats  together 
with  a  wire.  If  the  capacity  has  been  charged  to  a  suffi- 
ciently high  potential,  the  current  will  pass  between  the 
wire  and  the  coat  to  which  it  is  not  connected  providing 
this  gap  be  made  small  enough.  When  the  current  thus 
jumps  thru  the  air,  it  forms  the  electric  spark.  (The  phe- 
nomenon of  the  passage  of  the  electric  spark  will  be  taken 
up  under  the  subject  of  "  Spark  Gaps.")  This  discharge  of 
the  condenser  is  oscillatory.  That  is  to  say,  the  current 
does  not  simply  flow  from  one  plate  to  the  other  until  both 
are  charged,  or  discharged,  to  the  same  potential,  but 
continues  to  flow  back  and  forth  from  one  to  the  other  for 
a  considerable  length  of  time — relatively  speaking.  This 
phenomenon  is  most  easily  explained  by  the  U-tube  anal- 
ogy shown  in  Fig.  4.  If  one  side  of  the  tube  be  filled  with 
water,  and  the  pet-cock,  A,  at  the  bottom  be  opened,  the 
water  will  not  gradually  fill  up  the  other  side  of  the  tube 
until  the  level  in  both  arms  is  the  same,  but  will  rush,  or 
"  see-saw  "  back  and  forth  between  the  two  arms  of  the 
tube  until  finally  a  level  is  reached.  The  swinging  of  a 
pendulum  is  also  similar.  When  the  bob  is  drawn  to  one 
side,  it  acquires  potential  or  static  energy  by  virtue  of  the 

12 


INDUCTANCES.  [27 

force  of  gravity  which  tends  to  pull  it  down  to  the  vertical 
position.     This  is  equivalent  to  charging  the  condenser. 
When  the  bob  is  released,  corresponding  to  connecting  the 
two  coats  of  the  condenser  with  a  wire 
so  that  the  static  energy  (energy  at  rest 
—  resident  upon  the  condenser)  may  be 
converted  to  kinetic  energy  (energy  in 
motion),  the  pendulum  commences  to 
swing  back  and  forth,  eventually  coming 


to  rest.     If  the  resistance  of  the  wire  A 

connecting  the  two  coatings  of  the  con-  Fig.  4. 

denser  be  high,  the  current  will  oscillate 
but  a  few  times,  soon  coming  to  rest.  This  is  equiva- 
lent to  immersing  the  pendulum  in  a  liquid.  This  will 
tend  to  impede  or  damp  the  oscillations.  If  the  liquid 
should  be  very  thick,  the  pendulum  will  not  oscillate  but 
will  come  down  to  a  vertical  position  slowly.  A  very  high 
resistance  wire  will  similarly  cause  the  discharge  of  a  con- 
denser to  be  non-oscillatory.  The  principle  of  the  oscilla- 
tory discharge  of  a  condenser  should  be  fully  understood, 
since  it  is  the  basic  principle  on  which  the  production  of 
oscillatory  current  for  radio  transmitting  purposes  is 
founded. 

IV. 
INDUCTANCES. 

27.  If  a  current  of  electricity  is  passed  thru  a  coil  of 
wire,  lines  of  magnetic  flux  are  set  up  within  the  coil. 
These  lines  of  flux  or  magnetic  force  are  exactly  similar  to 
those  produced  by  an  ordinary  horse-shoe  or  bar  magnet. 
Within  the  coil,  the  lines  of  force  are  roughly  parallel  to  the 
axis  of  the  coil  —  a  line  running  thru  the  center  of  the  coil 
from  end  to  end.  Without  the  coil,  the  lines  of  force  flow 
from  one  end  to  the  other,  from  the  magnetic  North  pole 

13 


28]         ELEMENTS  OF  RADIOTELEGRAPHY. 

of  the  coil  to  the  South  pole.  A  simple  rule  by  which  to 
remember  the  relation  between  the  direction  of  the  cur- 
rent flow  within  the  coil  and  the  direction  of  the  magnetic 
lines  of  force  is  the  Fleming  right  hand  rule.  (After 
Prof.  J.  A.  Fleming,  a  modern  English  physicist  and  elec- 
trical engineer.)  If  the  right  hand  be  held  so  that  the 
thumb  is  at  right  angles  to  the  fingers,  and  the  fingers  be 
curved  so  as  to  point  in  the  direction  of  the  current  passing 
thru  the  convolutions  of  the  coil,  the  thumb  will  then  be 
pointing  in  the  direction  of  the  north  pole  of  the  coil,  and 
along  the  direction  of  the  lines  of  force  passing  thru  the 
center  of  the  coil.  Such  a  coil,  threaded  by  magnetic 
lines  of  force,  is  termed  an  inductance  coil.  The  induc- 
tance of  a  coil  may  be  defined  as  that  property  of  a  coil  by 
virtue  of  which  it  is  capable  of  storing  up  energy  in  electro- 
magnetic form.  The  formulae  for  inductances  are  very 
elaborate  and  are  subject  to  correction  factors.  It  is  con- 
sidered sufficiently  accurate  to  state  that  the  inductance  of 
a  coil  is  proportional  to  the  length  of  the  coil,  /,  the  square 
of  the  mean  diameter  of  the  turns,  c?,  the  square  of  the  num- 
ber of  turns  per  unit  of  length,  n,  and  depends  upon  the 
material  of  which  the  core  or  center  of  the  coil  is  made. 
A  core  of  soft  iron  will  give  a  much  greater  inductance  than 
one  of  wood  or  air. 

28.  The  unit  of  inductance  is  the  henry.     (After  Joseph 
Henry,  first  Secretary  of  the  Smithsonian  Institution,  and 
a  distinguished  American  physicist.)     The  henry  is  rather 
a  large  unit,  and  inductances  for  use  in  radiotelegraphy  are 
commonly  rated  in  millihenrys,  thousandths  of  a  henry. 
Inductances  may  be  measured  in  centimeters  as  well,  one 
millihenry  being  equal  to  1,000,000  cms. 

29.  In  connecting  inductances  in  series  or  parallel,  the 
same  rules  and  formulae  obtain  as  in  combining  resistances, 

14 


ELECTRO-MAGNETIC   INDUCTION.          [30 

so  that  equations  (5)  and  (7)  may  be  used  for  the  series 
and  parallel  connections  respectively  if  L  (the  symbol  for 
inductance)  be  substituted  for  R. 

V. 
ELECTRO-MAGNETIC   INDUCTION. 

30.  If  a  bar  magnet  be  suddenly  inserted  into  a  coil  of 
wire,  the  terminals  of  which  are  connected  to  a  galvanom- 
eter, which  is  a  sensitive  instrument  for  the  detection  of 
small  electric  currents,  the  indicating  needle  of  the  galvan- 
ometer will  be  observed  to  be  deflected  while  the  magnet 
is  being  inserted.  As  soon  as  the  magnet  is  no  longer 
moved,  the  needle  will  return  to  its  original  position.  If 
the  magnet  is  now  suddenly  removed,  another  deflection 
of  the  needle  will  occur  but  in  the  opposite  direction.  It 
is  apparent  that  in  the  act  of  inserting  the  magnet,  we  have 
generated  a  transient  current  of  electricity  in  the  coil,  and 
that  in  removing  it,  we  have  set  up  another  transitory  cur- 
rent of  electricity  in  the  opposite  direction.  This  phenom- 
enon is  known  as  the  electro-magnetic  induction  or  gener- 
ation of  electricity.  It  is  this  principle  which  is  employed 
in  the  production  of  current  by  the  modern  generator.  It 
should  be  noted  that  the  prime  requisite  for  this  method  of 
electrical  generation  is  motion.  So  long  as  the  magnet  is 
at  rest,  no  electricity  is  set  up.  The  magnet  is  surrounded 
by  lines  of  force,  and  when  moved,  these  lines  of  force  in 
cutting  or  passing  thru  the  turns  of  wire  produce  therein 
an  electromotive  force  sufficient  to  force  a  current  thru 
the  galvanometer.  The  potential  so  generated  is  propor- 
tional to  the  number  of  turns  of  wire  and  the  number  of 
lines  of  force  cut  per  second,  or  its  average  value  is 

N 

n  t  x  100,000,000 ' 

15 


31]         ELEMENTS   OF  RADIOTELEGRAPHY. 

where  E  is  the  E.M.F.  in  volts,  n  is  the  number  of  turns  in 
the  coil,  and  N  is  the  number  of  lines  of  force  cut  in  t  sec- 
onds. 

VI. 
ALTERNATING   CURRENT. 

31.  We  have  defined  direct  current  in  paragraph  15  as 
being  a  current  of  electricity  flowing  constantly  in  the  same 
direction  and  at  the  same  potential.  On  the  other  hand, 
alternating  current  is  a  current  of  electricity  which  is  con- 
tinually rising  or  falling  in  a  regular  manner  and  which 


changes  its  direction  of  flow  many  times  per  second.  Fig. 
5  shows  the  curve  of  potential  for  such  a  current.  The 
potential  starts  at  A  at  0  volts,  rises  gradually  to  100  volts, 
then  drops  to  0  volts  again.  At  this  point,  the  current  turns 
and  starts  to  flow  in  the  opposite  direction  in  the  circuit, 
the  potential  rising  to  100  volts  and  again  dropping  to  0 
volts,  after  which  the  whole  cycle  repeats  itself.  The 
change  of  direction  of  the  current  is  graphically  represented 
by  drawing  the  flow  in  one  direction  above  the  zero  line  and 
the  flow  in  the  opposite  direction  below  the  line.  From  A 
to  B  is  termed  one  alternation,  from  A  to  C  one  cycle. 
Thus,  a  cycle  consists  of  two  alternations.  The  number 
of  cycles  per  second  is  called  the  frequency.  For  ordinary 
commercial  practice,  60  cycle  current  is  common.  There 
are  thus  120  alternations  per  second,  and  in  Fig.  5,  the 
distance  from  A  to  C  would  represent  one  sixtieth  of  a 

16 


ALTERNATING  CURRENT.  [33 

second  ,  For  radio  transmitters  employing  alternating  cur- 
rent, 500  cycles  is  commonly  used  in  the  Navy.  Frequen- 
cies from  25  to  10,000  cycles  per  second  are  termed  audio 
frequencies,  since  they  lie  within  the  limits  of  audibility. 
Frequencies  from  10,000  to  1,000,000  and  more  are  termed 
radio  frequencies. 

32.  A  generator  producing  alternating  current  is  termed 
an  alternator.  It  generates  electricity  following  the  pro- 
cedure outlined  in  paragraph  30.  Lines  of  magnetic  force 
are  usually  set  up  by  field  windings  which  are  large  electro- 
magnets. These  field  poles  are  set  in  a  circle,  and  an  arma- 
ture composed  of  coils  of  wire  is  revolved  therein.  The 
voltage  thus  generated  may  be  computed  from  equation 
(13),  but  hi  this  case  instead  of  moving  the  magnet,  we 
are  moving  the  coil  of  wire.  The  net  result  is  of  course 
the  same  in  that  lines  of  magnetic  force  are  cut  by  the  coil. 
The  frequency  of  the  alternating  current  so  produced  is 
given  by  the  formula 


where  /  is  the  frequency  in  cycles,  P  is  the  number  of  field 
poles,  and  Rs  is  the  number  of  revolutions  per  second 
which  the  armature  makes.  Thus,  in  a  4-pole  alternator 
turning  over  1,800  rev.  per  min.,  the  frequency  would  be 
60  cycles. 

33.  We  have  seen  in  paragraph  30  that  a  rising  or  falling 
magnetic  field  within  a  coil  of  wire  will  set  up  therein  an 
E.M.F.  In  paragraph  30,  the  magnetic  field  was  set  up  by 
a  bar  magnet.  In  paragraph  27,  it  was  shown  that  a  cur- 
rent of  electricity  passing  thru  a  coil  of  wire  would  set  up 
a  magnetic  field  within  a  coil.  Obviously,  if  the  current 
in  the  coil  should  rise  and  fall,  the  magnetic  field  generated 

17 


34]         ELEMENTS  OF  RADIOTELEGRAPHY. 

thereby  would  also  rise  and  fall,  thus  fulfilling  the  condi- 
tion necessary  for  the  induction  of  an  E.M.F.,  within  the 
coil.  We  have  seen  in  paragraph  30  that  alternating  cur- 
rent is  a  rising  and  falling  current,  so  that  when  it  is  passed 
through  a  coil,  the  rising  and  falling  magnetic  field  sets  up 
an  induced  E.M.F.  This  E.M.F.  is  opposite  in  polarity  to 
the  E.M.F.  which  is  forcing  the  current  thru  the  coil,  with 
the  result  that  the  effective  potential  is  the  difference  be- 
tween the  applied  E.M.F.  and  the  induced  or  counter  E.- 
M.F. The  alternating  current  which  actually  passes  thru 
the  coil  is  thus  very  much  smaller  than  the  direct  current 
which  could  be  sent  thru  the  coil  with  the  same  applied 
E.M.F.,  for  the  direct  current  being  at  constant  potential, 
does  not  rise  and  fall,  its  magnetic  field  remains  constant, 
and  there  is  no  counter  E.M.F  induced.  This  property  of 
a  coil  to  impede  the  flow  of  alternating  current  is  called  its 
reactance.  The  amount  of  reactance  offered  to  an  alter- 
nating current  is  given  by  the  formula 

Xi  =  27T/L,  (15) 

where  X\  is  the  reactance,  TT  is  the  numeric  3.1416  (it  is 
that  number  which,  when  multiplied  by  the  diameter  of  a 
circle,  will  give  the  circumference  of  a  circle),  /  is  the  fre- 
quency in  cycles  per  second,  and  L  is  the  inductance  in 
henrys.  Reactance,  like  resistance,  is  measured  in  ohms. 

34.  From  equation  (15),  it  will  be  seen  that  an  increase 
in  the  frequency  results  in  an  increase  of  the  reactance 
which  any  particular  coil  may  have.  Reactance  coils  which 
will  permit  currents  of  audio  frequencies  to  pass  thru  them 
may  have  practically  an  infinite  reactance  to  those  of  radio 
frequencies.  As  the  frequency  is  decreased  more  and 
more,  the  reactance  of  a  coil  becomes  less.  Equation  (15) 
may  be  used  to  estimate  the  reactance  of  a  coil  on  direct 

18 


ALTERNATING  CURRENT.  [37 

current,  for  in  this  case  the  frequency  is  zero,  the  current 
flowing  constantly  in  one  direction  and  not  reversing,  and 
/  being  0,  the  reactance  is  thus  0.  A  coil  has  thus  no 
reactance  on  direct  current  but  on  both  alternating  and 
direct  current,  it  has  resistance  —  the  resistance  of  the 
wire  of  which  it  is  made. 

35.  The  reactance  of  a  condenser  is  called  capacity  or 
condensive  reactance  as  distinct  from  the  inductive  re- 
actance of  a  coil.  It  is  given  by  the  formula 


It  will  be  seen  that  contrary  to  the  action  of  an  inductance, 
the  reactance  of  a  condenser  decreases  with  an  increase  of 
frequency.  Since  /  is  hi  the  denominator  of  the  fraction,  a 
decrease  in  the  frequency  causes  the  reactance  of  the  con- 
denser to  rise  higher  and  higher  until  finally,  with  direct 
current,  when,  as  noted  in  paragraph  34,  the  frequency  is 
zero,  the  reactance  is  infinite.  A  condenser  on  a  direct 
current  circuit  is  thus  practically  an  open  circuit. 

36.  The  action  of  a  condenser  on  an  alternating  current 
circuit  is  thus  seen  to  be  exactly  the  reverse  of  that  of  an 
inductance,  in  fact,  the  reactance  of  a  coil  may  be  completely 
annulled  by  connecting  a  condenser  hi  the  circuit  with 
equal  reactance. 

37.  In  paragraph  16,  we  learned  that  the  relation  between 
potential,  current  and  resistance  was  given  by  Ohm's  law  as 

'.-I 

In  alternating  current,  besides  the  effect  of  resistance,  there 
is  also  the  effect  of  reactance,  as  we  have  just  seen,  which 

19 


37]         ELEMENTS  OF  RADIOTELEGRAPHY. 

tends  to  obstruct  or  impede  the  flow  of  current.  So  that 
Ohm's  law  must  be  changed  in  order  to  make  it  applicable 
for  alternating  current.  It  is  written  instead  as 


(17) 


where  Z  is  the  impedance  or  the  combined  effect  of  the 
resistance  and  the  reactance  —  both  inductive  and  capacity. 
Since  inductive  reactance  and  capacity  reactance  act  the 
reverse  of  each  other,  the  total  reactance  of  the  circuit,  X, 
is  equal  to  the  difference,  between  the  two,  or 

X  =  Xl  -  Xc.  (18) 

Thus,  it  there  were  10  ohms  of  inductive  reactance  and  6 
ohms  of  capacity  reactance,  the  total  reactance  would  be 
4  ohms  of  inductive  reactance,  since  the  latter  predomin- 
ated. In  other  words,  6  ohms  of  inductive  reactance  were 
taken  care  of  or  annulled  by  the  6  ohms  of  capacity  react- 
ance, leaving  4  ohms  of  inductive  reactance  effective.  The 
current  expended  in  setting  up  lines  of  force  in  a  coil,  ex- 
pended in  overcoming  the  reactance  of  the  coil,  is  not  ex- 
pended at  the  same  time  as  that  used  in  overcoming  the 
resistance  of  the  coil,  but  a  quarter  of  a  cycle  later.  Con- 
versely, the  current  used  in  charging  a  condenser,  over- 
coming the  reactance  of  a  condenser,  is  not  expended  at 
the  same  time  as  that  used  in  overcoming  the  resistance 
but  one  quarter  of  a  cycle  sooner.  Thus,  in  order  to  find 
the  impedance,  which  is  the  combination  of  the  resistance 
and  reactance,  the  resistance  cannot  simply  be  added  to 
the  resultant  reactance  (the  difference  between  the  induc- 
tive and  capacity  reactances).  Instead  they  are  combined 
at  right  angles,  as  shown  in  Fig.  6.  In  the  case  noted  in 
the  beginning  of  this  paragraph,  it  was  necessary  to  sub- 

20 


ALTERNATING   CURRENT. 


[38 


tract  the  capacity  reactance  of  6  ohms  from  the  inductive 
reactance  of  10  ohms,  leaving  a  resultant  of  4  ohms  induc- 
tive reactance.  This  is  shown  in  Fig.  7.  The  resistance 
R  is  shown  as  3  ohms.  Now,  when  two  forces  are  acting 


Fig.  6. 


together  at  right  angles,  their  resultant,  in  this  case  the 
impedance  Z,  is  obtained  by  making  a  rectangle  or  paral- 
lelogram as  shown  by  the  dotted  lines  and  by  taking  the 
diagonal  of  this  parallelogram.  This  principle  is  termed 
the  parallelogram  of  forces ,  and  holds  equally  true  whether 
the  forces  are  reactance  and  resistance,  or  the  speed  a  man 
may  be  making  in  a  row  boat  against  the  speed  of  the 
tide  at  right  angles  to  him. 

38.  We  thus  have  a  right  angled  triangle  ABC,  with  base 
3  and  height  4.  By  principles  of  geometry,  the  third  side 
or  hypotenuse  of  a  right  angled  triangle  is  equal  to  the 
square  root  of  the  sum  of  the  squares  of  the  other  two  sides. 
Thus 

Z  =  V#>  +  X\  (19) 

But  from  equation  (18), 

X  =  Xi  —  Xc, 

and  from  equation  (15), 

21 


39]         ELEMENTS   OF  RADIOTELEGRAPHY. 

Xi  =  27T/L, 
and  from  equation  (16), 


"2T/C' 

so  that  equation  (19)  may  be  written 


Z  =  V*2  +     27T/L  -  -—       .  (20) 


In  the  case  shown  in  Fig.  7, 


z  =    a2  +  42 


-  A9  +  16 

=  V25 
=  5. 
Thus  the  impedance,  7,  equals  5  ohms. 

39.  We  have  seen  in  paragraph  36,  that  the  reactance 
of  a  coil  may  be  annulled  by  that  of  a  condenser  providing 
they  are  equal,  as  when 


But  if  this  condition  obtains  in  equation  (20),  then  that  part 
of  the  formula  in  brackets  will  cancel  leaving  0.  O2  equals 
0,  so  that  when  the  inductive  reactance  equals  the  capacity 
reactance, 

Z  =  R.  (22) 

When  these  two  reactances  are  equal,  a  state  of  resonance 
is  said  to  obtain.  An  alternating  current  circuit  in  such  a 
case  would  be  exactly  similar  to  a  direct  current  circuit, 

22 


ALTERNATING   CURRENT.  [41 

i.e.,  there  would  be  no  reactance,  and  Ohm's  Law,  as  given 
in  equation  (1),  would  apply. 

40.  When  there  is  reactance  in  a  circuit,  the  current  is 
not  in  phase  with  the  impressed  voltage,  that  is,  it  does  not 
reach  its  maximum  or  minimum  values  at  the  same  time 
as  does  the  potential.    If  there  is  a  predominance  of  in- 
ductive reactance,  it  will  lag  behind  the  potential.     If  there 
is  more  condensive  than  inductive  reactance,  the  current 
will  lead  the  potential  —  it  will  reach  its  maximum  values 
before  the  potential  does.     The  same  angular  or  phase  re- 
lationship obtains  between  the  potential  and  the  current  as 
does  between  the  resistance  and  the  impedance.    In  Fig. 
7,  the  angle  between  the  resistance  and  impedance,  BA  C, 
called  by  the  Greek  letter  theta  —  6  —  is  also  the  angle  of 
phase  displacement  between  the  potential  and  current. 
The  current  is  said  to  lag  0  degrees  behind  the  voltage. 
By  trigonometry,  the  value  of  this  angle  may  be  computed. 

41.  The  side  opposite  to  the  angle  0,  BC,  divided  by  the 
hypotenuse,  AC,  is  termed  the  sine  of  the  angle  0.    Thus 

BC 

^  =  *m0.  (23) 

The  side  adjacent  to  the  angle  0,  AB,  divided  by  the  hypot- 
enuse, AC,  is  termed  the  cosine  of  the  angle  0.     Or 


(24) 


The  side  opposite  the  angle  0,  £C,  divided  by  the  side  ad- 
jacent to  the  angle  0,  AB,  is  termed  the  tangent  of  the  angle 

0,  or 

BC 

-r^  =  tan  0.  (25) 

AD 

23 


42]         ELEMENTS  OF  RADIOTELEGRAPHY. 

4 

In  the  case  noted  in  Fig.  7,  tan  6  =  - 

o 

=  1.333. 

By  consulting  a  table  of  tangents,  1.333  is  found  to  be  the 
tangent  of  the  angle  of  53°  8'.  This  angle  could  be  simi- 
larly found  by  taking  the  sine  or  cosine  of  the  angle,  since 
all  three  sides  of  the  triangle  in  this  case  are  known.  The 
current  thus  lags  at  an  angle  of  53°  8'  behind  the  voltage. 

42.  In  equation  (2),  it  was  noted  that  the  power  in  a 
direct  current  circuit  is  equal  to  the  voltage  times  the  cur- 
rent. In  an  alternating  current  circuit  containing  react- 
ance, this  formula  does  not  give  the  true  power  but  the  ap- 
parent power.  It  is  ob- 
vious that  since  the  cur- 
rent and  potential  are  not 
E  acting  in  unison  or  phase, 

~~>     their  joint  effort  or  power 
Fig  8  cannot  be  the  product  of 

the  two  as  in  the  case  of 

direct  current  when  they  are  always  in  phase.  In  Fig.  8, 
that  portion  of  /  which  is  in  phase  with  the  voltage  E  may 
be  considered  as  the  projection  of  AC  on  E  which  is  AB,  or 
we  may  say  that  AB  is  that  portion  of  the  current  I  which  is 
in  phase  with  E.  The  formula  for  power  would  then  be 

P  =  E(AB),  (26) 

which  is  similar  to  equation  (2).     From  (24) 
AB 

Tc  =  cose. 

But  since  AC  represents  the  current  /,  we  may  write 

24 


ALTERNATING   CURRENT.  [44 

AB 

—  =  cos  0, 

or 

AB  =  I  cos  B.  (27) 

Substituting  /cos  0  for  AB  in  equation  (26),  we  have 

P  =  El  cosB,  (28) 

which  is  the  formula  for  power  in  alternating  current,  the 
angle  6  being  the  angle  of  lead  or  lag  as  the  case  may  be. 
The  expression  cos  0  is  termed  the  power  factor.  It  is 
that  expression  which  when  multiplied  by  the  apparent 
watts,  £7,  gives  the  true  watts.  The  power  factor,  cos  0, 
is  never  greater  than  unity. 

43.  When  there  is  no  reactance  in  the  circuit,  or  when 
the  condensive  reactance  balances  or  equals  the  inductive 
reactance  so  that  the  resultant  reactance  is  nil,  the  current 
is  in  phase  with  the  voltage  and  the  angle  of  phase  dis- 
placement is  zero.     The  cosine  of  zero  is  unity — 1 — which, 
when  substituted  in  equation  (28),  gives  us  the  same  equa- 
tion for  power  for  alternating  current  as  for  direct  current. 
It  is  correct  then  to  state  that  when  an  alternating  current 
circuit  is  in  a  state  of  resonance,  as  defined  hi  paragraph 
39,  the  same  formulae  obtain  as  for  direct  current  circuits. 

44.  In  the  case  noted  in  paragraph  38,  we  found  the  im- 
pedance to  be  5  ohms,  and  that  the  current  lagged  53°  8' 
behind  the  voltage.     Assume  a  potential  of  100  volts  across 
the  circuit,  represented  by  E  in  Fig.  8.     What  is  the  value 
of  the  current  7?     What  is  the  power  expended  in  the  cir- 
cuit?    From  equation  (17),  we  have 

7=^=20,         ' 

5 

or  the  current  is  20  amperes.     The  cosine  of  53°  8'  is  0.60. 
3  25 


44]         ELEMENTS  OF  RADIOTELEGRAPHY. 

(We  say  that  the  power  factor  is  60%.)  The  power,  real 
watts,  may  be  computed  from  equation  (28)  as  P  =  100 
X  20  X  0.6  =  1,200.  The  power  in  real  watts  is  thus 
1,200  watts,  the  apparent  wattage  is  100  X  20  or  2,000 
watts. 


26 


CHAPTER  TWO. 

vn. 

DAMPING  AND   RESONANCE. 

45.  In  paragraph  26,  it  was  observed  that  the  discharge 
of  a  condenser  was  oscillatory,  that  is  to  say,  the  discharge 
current  oscillated  back  and  forth  in  the  circuit  until  it  even- 
tually died  out,  similar  to  the  gradual  dying  out  of  the  swings 
of  a  pendulum.    In  other  words,  the  discharge  of  a  con- 
denser gives  rise  to  the  production  of  an  alternating  cur- 
rent (even  tho  it  may  be  charged  with  direct  current),  only, 
unlike  the  alternating  potential  shown  in  Fig.  5,  the  maxi- 
mum value  of  which  is  100  volts  in  either  direction,  the 
maximum  potentials  for  an  alternating  current  set  up  by 
the  discharge  of  a  condenser  gradually  grow  less  until 
they  finally  reach  zero.     This  is  shown  in  Fig.  9.     Such  a 
current  or  wave  is  called  a  damped  wave.     (To  damp  the 
swinging  or  vibration  of  a  body  is  to  check  its  vibration.) 
A  wave  similar  to  that  in  Fig.  5  is  termed  an  undamped 
wave  for  its  maximum  values  are  not  reduced — the  swing- 
ing of  this  alternating  current  is  not  checked.     In  radio- 
telegraphy,  all  forms  of  transmitters  employing  sparks  give 
rise  to  damped  waves,  while  the  Poulsen  arc  and  alter- 
nators of  radio  frequency,  such  as  the  Alexanderson  and 
Goldschmidt    generators,    radiate    practically    undamped 
waves.     To  understand  the  effect  of   damping,  it  will  be 
necessary  to  consider  the  phenomenon  of  resonance  be- 
tween two  vibrating  bodies. 

46.  A  definition  of  resonance  is  very  excellently  set  forth 
in  an  opinion  rendered  by  Judge  Veeder  of  New  York  in  a 
radio  patent  suit  as  follows : 

27 


46]         ELEMENTS   OF  RADIOTELEGRAPHY. 

"Resonance  is  an  increase  of  amplification  of  a  periodic "  (vibrating) 
"motion  by  an  intermittent  force  of  the  same  frequency.  A  certain  or 
natural  period  of  vibration  is  characteristic  of  all  bodies  which,  when 
displaced  by  the  application  of  external  force,  tend  by  virtue  of  their 
elasticity,  to  return  and  to  execute  free  vibrations  until,  by  virtue  of 
their  inertia,  they  gradually  come  to  rest.  Sonorous  bodies,  such  as 
string  under  tension,  and  confined  portions  of  air,  as  in  the  organ  pipe, 
are  further  illustrations  suggested  by  the  term.  Just  as  very  feeble 
impulses  applied  to  a  pendulum  at  rest,  at  intervals  exactly  correspond- 
ing to  its  natural  period  of  vibration,  will  cause  almost  any  desired 
amplitude  of  swing,  so  bodies  capable  of  executing  vibrations  by  virtue 
of  their  own  resiliency  may  be  put  into  strong  vibration  by  a  series 
of  impulses  in  tune  with  their  own  natural  period.  Thus  impulses  from 
a  tuning  fork  will  cause  another  tuning  fork  of  the  same  pitch  to  hum  a 
note  in  unison. 

"  Resonance  effects  may  likewise  be  observed  in  the  flow  of  elec- 
tricity in  a  circuit.  A  circuit  possessing  inductance  and  capacity  has  a 
certain  time  period  of  vibration ;  that  is,  it  takes  a  certain  length  of 
time  for  an  oscillation  to  complete  itself  in  the  circuit.  Such  a  circuit 
is  said  to  have  a  definite  wave  length.  A  circuit  possessing  capacity 
and  inductance  tends  to  oscillate  at  its  own  frequency.  It  becomes 
the  seat  of  an  induced  oscillatory  current  when  subjected  to  the  influ- 
ence of  electric  waves  of  that  frequency,  each  wave  giving  a  slight  im- 
pulse to  the  oscillations  already  excited,  with  the  result  that  the  induced 
electromotive  forces  will  be  amplified  in  intensity,  just  as  the  swing  of 
a  pendulum  is  increased  by  the  application  of  properly  tuned  tho  feeble 
touches.  However,  not  only  must  the  impulses,  of  whatever  kind,  be 
rightly  timed,  but  it  is  also  essential  to  the  utilization  of  resonance  that 
there  should  be  a  long  series  of  such  impulses  of  approximately  equal 
strength  or  amplitude.  Having  regard  to  ether  waves,  such  a  train 
can  only  result  where  oscillations  from  which  they  proceed  occur  in  a 
circuit  which  gives  out  its  energy  slowly,  for  the  amplitude  of  the 
waves  depends  upon  the  energy  expended. 

"  The  energy  must  not  be  wasted  either  by  internal  resistance  or  by 
lavish  radiation.  On  the  other  hand,  if  ether  waves  are  to  be  detected 
at  any  great  distance,  they  must  be  of  substantial  amplitude.  Hence, 
a  circuit  which  is  a  good  conserver  of  energy,  capable  of  creating  a 
train  of  waves  of  approximately  equal  amplitude,  is  a  feeble  radiator; 
conversely,  a  good  radiator,  giving  out  its  energy  in  one  big  wave  fol- 
lowed by  rapid  damping  of  the  remainder,  is  obviously  a  poor  conserver 
of  energy  and  will  not  create  the  necessary  train  of  waves  of  approxi- 

28 


DAMPING  AND   RESONANCE.  [48 

mately  equal  amplitudes.  A  similar  difficulty  applies  to  the  detector. 
A  good  radiating  circuit  will  be  a  good  absorbing  circuit,  but  it  will  not 
be  a  good  conserver.  In  proportion  to  its  susceptibility  to  ether  waves 
is  its  unfitness  to  accumulate  a  train  of  waves,  for  it  tends  to  give  off 
forthwith  the  energy  it  receives." 

47.  An  additional  explanation  of  the  phenomenon  of  res- 
onance set  forth  in  the  above  may  be  found  helpful.     If  a 
tuning  fork  be  set  into  vibration  by  striking  one  of  its 
prongs  it  will  send  out  a  wave  the  frequency  of  which  corre- 
sponds to  the  pitch  of  the  fork.     The  pitch  of  the  fork  de- 
pends upon  its  physical  construction,  i.e.,  the  length  of  the 
prongs   and   their   thickness   and   weight.     These   sound 
waves  striking  the  prongs  of  another  tuning  fork  set  the 
latter  into  vibration.     This  will  not  occur,  however,  unless 
the  first  fork  radiates  a  succession  of  waves,  each  of  which 
is  of  practically  the  same  strength  or  amplitude.     Such 
a  train  of  waves  of  feeble  damping  (slowly  decreasing 
amplitude)  will  build  up  in  the  second  fork  waves  of  grad- 
ually increasing  amplitude.     If,  on  the  other  hand,  the 
waves  from  the  first  fork  should  be  highly  damped  (come 
to  rest  quickly)  no  appreciable  effect  will  be  made  on  the 
latter.     Thus  we   see  that  feeble   damping, — slowly  de- 
creasing amplitude  of  the  waves,  persistency  of  oscillation 
—is  necessary  if  the  principle  of  resonance,  i.e.,  the  setting 
up  of  vibrations  in  one  body  from  waves  sent  out  by  an- 
other vibrating  body  of  the  same  pitch  or  wave  length,  is 
to  be  utilized. 

48.  In  radiotelegraphy,  the  first  tuning  fork  corresponds 
to  the  transmitting  station — the  second  tuning  fork  to  the 
receiver.     In  order  that  the  receiver  may  be  set  into  oscil- 
lation by  waves  from  the  transmitter  they  must  be  tuned  to 
the  same  pitch  or  wave  length.     And  in  order  that  a  maxi- 
mum advantage  may  be  taken  of  this  principle  of  reson- 
ance, it  will  be  necessary— as  set  forth  in  the  preceding 

29 


49]         ELEMENTS   OF  RADIOTELEGRAPHY. 

paragraphs — for  the  transmitted  wave  to  be  as  feebly 
damped  as  possible.  If  only  one  single  wave  were  radi- 
ated from  the  transmitter  instead  of  a  succession  of  waves 
of  practically  constant  intensity  or  strength,  there  could  be 
no  accumulative  or  adding  up  effect  in  the  receiver,  and 
signals  could  not  be  received. 

49.  Consequently,  in  designing  a  successful  radio  trans- 
mitter, we  must  endeavor  to  produce  waves  from  the  antenna 
of  as  low  a  damping  as  possible  and  to  provide  means 
at  the  receiver  for  tuning  its  period  to  that  of  the  sending 
station.     Before  tracing  the  efforts  of  vaiious  inventors  to 
attain  this  end,  we  shall  spend  a  short  time  in  an  investi- 
gation of  what  factors  contribute  to  low  or  high  damping 
of  the  current  in  the  antenna  circuit  of  the  transmitter. 

VIII. 
LOGARITHMIC    DECREMENT. 

50.  The  damping,  or  the  rate  of  decay  or  decrease,  of  a 
damped  alternating  current  is  expressed  mathematically 
by  its  logarithmic  decrement,  but  to  understand  this  latter 
term  it  will  be  necessary  to  approach  the  subject  of  loga- 
rithms. 

51.  Ten  squared  equals  100.     It  is  written  mathematic- 
ally as  follows : 

102  =        100. ' 

Similarly  103  =     1,000,   •  (29) 

and  104  =  10,000. . 

We  say  that  100  is  the  second  power  of  10,  1,000  is  the 
third  power  of  10,  and  10,000  is  the  fourth  power  of  10. 
10  is  of  course  the  first  power  of  10.  We  say  that  in  equa- 

30 


LOGARITHMIC   DECREMENT.  [53 

tions  (29),  2,  3,  and  4  are  exponents  of  10.  As  we  think  of 
10  as  being  the  first  power  of  10,  of  100  as  being  the  2d 
power  of  10,  of  1,000  as  being  the  3d  power  of  10,  and  so 
on,  so  we  may  consider  any  number  as  being  a  power  of 
ten.  The  numbers  lying  between  10  and  100  will  be 
somewhere  between  the  first  and  second  powers  of  ten. 
Those  numbers  between  100  and  1,000  will  be  more  than 
the  second  power  of  ten  and  less  than  the  third  power. 
Tables  have  been  prepared  by  which  it  is  possible  to 
obtain  that  power  of  ten  which  will  raise  it  to  any  given 
number.  Thus  101  equals  10.  We  say  that  the  logarithm 
of  10  to  the  base  10  is  1.  Similarly,  the  logarithm  of  100 
is  2,  and  the  logarithm  of  1,000  is  3.  We  should  expect 
the  logarithm  of  45  to  be  greater  than  1  and  less  than  2. 
From  a  table  of  logarithms,  we  find  it  to  be  1.65321,  or 

101.65321  euals  45 


52.  It  is  not  necessary  to  use  10  as  the  base  of  a  system 
of  logarithms.     That  is  the  most  convenient,  for  our  sys- 
tem of  counting,  the  Arabic,  is  also  based  on  10.     However, 
any  number  may  be  chosen  as  the  base  for  a  system  of 
logarithms,  3  for  example.     With  3  as  a  base,  the  logarithm 
of  9  would  be  2,  log  27  would  be  3,  log  81  would  be  4,  and 
so  on,  for  9  is  the  2d  power  of  3,  27  is  the  3d  power  of  3, 
and  81  is  the  4th  power  of  3. 

53.  That  system  of  logarithms  which  uses  10  for  a  base 
is  called  the  common  or  Briggs  system  of  logarithms  (after 
the  mathematician  who  first  worked  them  out)  .     However, 
there  is  also  another  system  of  logarithms  used  in  ad- 
vanced mathematics  termed  the  natural  or  Napierian  sys- 
tem of  logarithms.     The  base  of  this  system  instead  of 
being  10  is  2.7183.     In  other  words,  all  numbers  instead  of 
being  considered  as  certain  powers  of  10  are  represented  as 
exponents  of  powers  of  2.7183. 

31 


54]         ELEMENTS   OF  RADIOTELEGRAPHY. 

54.  The  logarithm  of  a  number  may  thus  be  defined  as 
the  exponent  of  the  power  to  which  a  fixed  number,  the 
base,  must  be  raised  in  order  to  produce  that  number.     In 
speaking  of  logarithms  in  connection  with  the  measure- 
ment of  damping,  we  have  reference  to  the  natural  system, 
i.e.,  to  the  base  2.7183,  and  not  to  the  common  system. 

55.  In  Fig.  9,  it  will  be  noted  that  the  decreasing  cur- 
rent amplitude  does  not  die  out  following  a  straight  line, 


High  Damping 


Fig.  9. 

but  according  to  a  curve,  as  drawn  in  the  dotted  lines. 
Such  a  curve  is  called  a  logarithmic  or  exponential  curve 
and  it  is  customary  to  measure  the  rate  or  rapidity  of 
damping  by  taking  the  difference  between  the  logarithm 
of  the  height  of  one  oscillation  and  the  logarithm  of  the 
height  of  the  next  succeeding  oscillation  in  the  same 
direction.  Thus  the  logarithmic  decrement,  5,  of  the 
current  shown  in  Fig.  9  is  measured  by  taking  the  differ- 
ence between  the  logarithm  of  AB  and  that  of  CD,  or 

d  =  log  AB  -  log  CD.  (30) 

By  the  theory  of  logarithms,  equation  (30)  may  be  written 


32 


LOGARITHMIC   DECREMENT.  [57 

or  the  logarithmic  decrement  equals  the  natural  logarithm 
of  the  ratio  of  the  height  of  one  oscillation  to  that  of  the 
next  succeeding  oscillation  in  the  same  direction.  Loga- 
rithmic decrement  is  thus  a  measurement  of  damping. 

56.  In  an  alternating  current  circuit,  the  resistance,  in- 
ductance and  capacity  all  affect  the  damping  or  logarithmic 
decrement  as  given  by  the  formula 


6  =  7rtf^.  (32) 

This  formula  is  strictly  applicable  only  to  those  circuits 
which  are  non-radiative,  but  if  R  represents  the  total  re- 
sistance of  the  antenna  circuit,  ohmic  and  radiation  resist- 
ance, the  formula  is  accurate  (See  par.  305).  From  this 
formula,  it  will  be  seen  that  the  greater  the  resistance  and 
the  capacity  of  a  circuit  and  the  less  the  inductance,  the 
greater  will  be  the  damping  or  the  decrement  of  the  cir- 
cuit.* In  order,  then,  to  produce  feeble  damping  of  the 
current  in  the  antenna  circuit  and  hence  feeble  damping 
of  the  electrical  waves  which  it  radiates,  which  is  what  we 
desire  to  do  as  set  forth  in  the  preceding  paragraphs,  we 
must  have  a  high  value  of  inductance  in  the  circuit  and 
low  values  of  resistance  and  capacity. 

57.  Circuits  which  are  so  designed  as  to  give  alternating 
current  of  feeble  damping  are  termed  persistently  oscillat- 

*  In  certain  alternating  current  circuits  in  which  the  properties  of 
the  circuit  are  not  constant,  the  decrement  of  oscillations  therein  is 
linear  and  not  logarithmic.  That  is  to  say,  the  dotted  lines  shown  in 
Fig.  9  will  be  straight  lines  in  place  of  the  logarithmic  curves  shown. 
Such  oscillations  occur  in  spark  gap  circuits  in  which  the  resistance 
of  the  gap  is  made  to  rise  very  rapidly.  Linear  decrement  is  obviously 
measured  by  the  difference  between  the  actual  heights  of  two  suc- 
ceeding oscillations  in  the  same  direction  instead  of  the  logarithms  of 
those  heights. 

33 


58]         ELEMENTS  OF  RADIOTELEGRAPHY. 

ing  circuits,  that  is  to  say,  the  oscillations  are  persistent, 
they  do  not  quickly  subside. 

58.  In  paragraph  47,  we  observed  that  in  order  to  make 
use  of  the  principle  of  resonance  between  the  transmitter 
and  receiver,  which  is  the  method  by  which  energy  in  the 
latter  is  set  up  by  waves  from  the  former,  it  is  necessary 
to  have  feebly  damped  waves.     When  full  use  of  the  phe- 
nomenon of  resonance  is  so  made,  we  say  that  the  tuning 
at  the  receiver  is  sharp,  that  is  to  say,  signals  from  the  trans- 
mitter can  only  be  heard  at  but  one  setting  of  the  receiver. 
This  is  desirable,  because  if  signals  from  any  one  station 
are  heard  over  a  wide  range  of  wave  lengths  (broad  tuning), 
interference  results,  since  it  is  not  possible  to  tune  out  a 
station  not  desired  in  order  to  receive  from  a  station  from 
which  signals  are  desired  to  be  read. 

59.  To  remedy  this  interference  evil,  the  United  States 
Government  in  1912  adopted  a  law  which,  among  other  pro- 
visions, required  that  the  logarithmic  decrement  of  the  waves 
radiated  from  a  transmitter  should  not  exceed  two-tenths. 
Or  referring  to  Fig.  9  and  equation  (30),  log  AB  —  log  CD 
should  not  be  greater  than  0.2.     With  such  a  decrement, 
a  train  of  waves  has  12.5  waves  in  it  before  the  amplitude 
of  the  oscillations  drops  to  one  tenth  of  that  of  the  first 
oscillation  or  current  swing.     If  the  decrement  be  less  than 
0.2,  there  are  more  waves  in  the  train,  so  that  with  a  loga- 
rithmic decrement  of  0.04,  which  is  not  uncommon  with 
the  quenched  gap  transmitters  in  use  in  the  Navy,  there 
are  58  oscillations  in  a  wave  train  before  the  amplitude  of 
the  oscillations  falls  to  one  tenth  of  that  of  the  first  oscilla- 
tion in  the  train.     With  58  complete  current  swings  strik- 
ing the  receiving  antenna,  full  advantage  may  be  taken 
of  the  principle  of  resonance  as  set  forth  in  paragraph  47, 
and  very  sharp  tuning  results.     For  sharp  tuning,  then,  we 

34 


WAVE  LENGTH,  FREQUENCY,  TIME  PERIOD.    [61 

must  have  a  wave  of  low  logarithmic  decrement — the  waves 
sent  out  from  the  transmitting  antenna  must  die  out  grad- 
ually so  that  there  will  be  a  long  series  of  waves  in  each 
train  of  only  slightly  decreasing  amplitude. 

IX. 

WAVE  LENGTH,  FREQUENCY,   TIME  PERIOD 

60.  While  we  have  learned  the  formulae  governing  the 
rapidity  of  decay  of  the  alternating  current  in  a  circuit,  i.e., 
the  damping,  it  is  also  necessary  to  know  the  formulas  for 
the  wave  length  and  other  factors  in  connection  with  oscil- 
latory currents. 

61.  The  discharge  of  a  condenser  as  described  in  para- 
graph 26  of  Chapter  One,  is  called  a  free  oscillation  as  dis- 
tinct from  a  forced  oscillation.     With  free  oscillations,  we 
may  consider  that  a  certain  charge  is  given  to  a  circuit 
which  is  converted  into  energy  in  motion.     The  circuit  is 
not  driven  into  oscillation,  it  is  allowed  to  oscillate.     The 
oscillations  of  a  pendulum  which  is  drawn  to  one  side  and 
then  released  are  free  oscillations,  the  frequency  of  the 
pendulum  swings  being  determined  solely  by  the  length  of 
the  pendulum.     But  if  the  pendulum  were  grasped  by  the 
hand  and  made  to  swing  according  to  the  swings  of  the 
hand,  its  oscillations  would  then  be  forced  oscillations  for 
the  frequency  of  its  swings  would  not  then  be  determined 
by  its  own  dimensions  but  by  the  characteristics  of  the 
swinging  force — the  frequency  of  the  hand  swings.     Thus, 
a  forced  alternating  current  is  defined  by  the  1915  Standar- 
dization Report  of  the  Institute  of  Radio  Engineers  as  "  a 
current,  the  frequency  and  damping  of  which  are  equal  to 
the  frequency  and  damping  of  the  exciting  electromotive 
force."     This,  it  will  be  seen,  is  exactly  similar  to  the  pen- 

35 


62]         ELEMENTS  OF  RADIOTELEGRAPHY. 

dulum  analogy  just  given.  The  alternating  current  in  a 
commercial  lighting  circuit  may  be  considered  as  forced, 
undamped  oscillations,  since  the  frequency  and  damping 
of  these  oscillations  is  that  of  the  applied  alternating  E.M.F. 
at  the  terminals  of  the  circuit.  No  matter  how  much  re- 
sistance, inductance  or  capacity  there  may  be  in  the  cir- 
cuit, the  current  oscillations  or  cycles  will  always  have  the 
frequency  of  the  applied  E.M.F.—  they  are  distinctly  forced 
oscillations. 

62.  In  paragraph  26  of  Chapter  One  we  observed  that 
if  the  resistance  of  the  wire  connecting  the  two  coatings  of 
the  condenser  to  be  discharged  were  high,  the  current 
would  oscillate  but  a  few  times,  while  if  it  were  very  high, 
it  would  not  oscillate  at  all  but  the  potential  to  which  the 
condenser  was  charged  would  slowly  drop  to  zero.  Thus, 
we  see  that  in  order  to  have  free  oscillations  in  a  circuit 
the  value  of  the  resistance  is  limited.  Its  value  is  deter- 
mined by  the  amount  of  inductance  and  capacity  in  the 
circuit  for  we  have  found  in  equation  (32)  that  while  the 
resistance  and  capacity  tends  to  damp  out  or  stop  the  os- 
cillations, the  inductance  tends  to  prolong  them.  This 
is  expressed  mathematically  as  follows:  If  the  resistance 
of  a  circuit  be  less  than  twice  the  square  root  of  the  induc- 
tance divided  by  the  capacity,  that  is,  if 


R  <  2  \     ,  (33) 

there  will  be  free  oscillations  or  a  free  alternating  current 
in  the  circuit.  If,  however,  the  resistance  should  be  greater 
than  this  amount,  that  is  to  say,  if 


(34) 
36 


WAVE  LENGTH,  FREQUENCY,  TIME  PERIOD.     [64 

the  circuit  will  not  oscillate.  It  is  equivalent  to  the  case 
in  paragraph  26  in  which  the  pendulum  was  immersed  in 
a  liquid  so  thick  that  it  could  not  oscillate.  The  resistance 
or  friction  of  the  liquid  was  thus  too  great  for  a  pendulum 
of  that  particular  length  to  permit  it  to  swing.  Equations 
(33)  and  (34)  thus  state  in  different  fashion  the  facts  set 
forth  in  equation  (32),  i.e.,  the  greater  the  resistance  and 
the  capacity,  the  fewer  the  oscillations,  the  higher  the 
damping,  while  the  larger  the  inductance,  the  greater  the 
number  of  oscillations  and  the  less  the  damping. 

63.  In  addition  to  affecting  the  total  number  of  oscilla- 
tions or  cycles  of  free  alternating  current,  the  inductance 
and  capacity  also  determine  the  number  of  oscillations  per 
second  or  their  frequency.  The  relation  is  given  as  follows  : 


(3S) 


where  /  represents  the  frequency  or  number  of  cycles  per 
second,  ir  is  the  numeric  3.1416,  C  is  the  capacity  and  L  is 
the  inductance. 

64.  The  time  which  :s  consumed  while  one  oscillation 
completes  itself  in  the  circuit  is  termed  the  time  period  of 
the  circuit  and  is  determined  by  the  inductance  and  capa- 
city. It  is  obvious  that  the  longer  time  it  takes  for  an  os- 
cillation to  complete  itself,  the  fewer  oscillations  there  can 
be  per  second  or  the  lower  the  frequency,  and  vice  versa. 
Hence,  the  time  period  varies  inversely  as  the  frequency 
and  is  the  reciprocal  of  the  frequency  or 


T  =  27r  Vc  (36) 

where  T  represents  the  time  period.     This  means  that  one 
wave  will  follow  the  preceding  one  from  the  antenna  every 

37 


65]         ELEMENTS  OF  RADIOTELEGRAPHY. 


seconds.  The  distance  away  that  one  wave  will 
get  from  the  antenna  before  the  next  one  follows  will  of 
course  depend  upon  its  velocity,  so  that  the  wave  length 
or  the  distance  between  one  wave  and  the  next  preceding 
or  succeeding  wave  will  equal  the  velocity  times  the  time 
elapsing  between  successive  waves  or 


X  =  2TTV    c,  (37) 

where  X,  the  Greek  letter  lambda,  represents  the  wave 
length  and  v  equals  the  velocity  of  the  radio  wave  and  of 
light  which  we  found  in  paragraph  10  to  be  300,000,000 
meters  per  second. 

65.  If  the  inductance  be  measured  in  centimeters,  see 
paragraph  28,  and  the  capacity  in  microfarads,  see  para- 
graph 24,  the  formula  for  wave  length  in  meters  becomes 

X  =  59.6  VcZ.  (38) 

66.  Since  the  alternating  current  we  have  been  noting 
above  is  oscillating  freely,  we  should  expect  it  to  oscillate 
at  that  frequency  which  will  give  the  least  impedance  to 
the  current.    In  paragraph  37,  we  defined  the  impedance 
of  an  alternating  current  circuit  to  be  the  combination  of 
the  resistance  and  the  reactance,  both  inductive  and  con- 
densive.    The  resistance  cannot  vary  with  the  frequency, 
but  the  reactance,  as  seen  from  equations  (15)  and  (16), 
can.     That  frequency  which  will  make  the  inductive  react- 
ance of  the  circuit  equal  to  the  capacity  reactance  and  thus 
lower  the  impedance  value  to  merely  that  of  the  resistance 
as  set  forth  in  paragraph  39,  is  the  frequency  at  which  the 
free  alternating  current,  whose  wave  length  is  given  by 
equation  (37),  oscillates.     This  may  be  proven  by  equating 
equations  (15)  and  (16),  which  are  respectively: 

38 


WAVE  LENGTH,  FREQUENCY,  TIME  PERIOD.    [67 

Xi  =  27T/L, 
1 

C    ~    27T/C' 

Thus, 

2^  =  ^c: 

clearing  of  fractions 

47r2/2CL  =  1, 
or 


and  consequently  the  frequency  is 


and  the  wave-length  is 


X  =     =  2irv 


which  was  to  be  proved.    This  demonstration  assumes 
that  the  damping  is  not  very  great. 

67.  In  receiving  signals  at  a  receiving  station  from  a 
transmitter,  we  have  an  alternating  induced  E.M.F.  set  up 
in  the  receiving  antenna  from  the  incoming  waves,  the 
frequency  of  which  is  that  of  the  waves.  To  get  a  maxi- 
mum flow  of  current  hi  the  antenna  circuit  of  the  receiver 
with  consequent  maximum  strength  of  signals,  we  must 
have  the  impedance  of  the  circuit  as  low  as  possible,  as 
set  forth  in  equation  (17).  To  reduce  the  impedance  to  the 
lowest  possible  limit,  the  resistance,  we  must  have  the  in- 
ductive and  condensive  reactances  equal  or  the  circuit  in 
resonance  as  set  forth  hi  paragraph  39.  Thus  in  tuning  the 

39 


68] 


ELEMENTS   OF  RADIOTELEGRAPHY. 


receiver  to  the  incoming  wave,  we  must  adjust  the  in- 
ductance and  capacity  of  the  circuit  so  that  their  reactances 
will  balance  each  other,  when  the  receiver  will  be  in 
resonance  with  the  transmitter  as  set  forth  in  paragraphs 
45  to  49.  So  that  the  two  definitions  of  resonance  as  we 
have  seen  them  applied  to  this  subject  are  in  accord.  (The 


Capacity          Inductance 


Variable 


Hlllllllr- 


Resistance 


Spark  Gap 


Oscillation  Transformer 
or  Antenna  Coupler 


conditions  noted  in  the  beginning  of  this  paragraph  are 
quite  accurate  when  the  damping  is  not  too  great.) 

68.  Having  made  a  brief  study  of  the  subject  of  damp- 
ing and  its  effect^  on]  resonance,  we  may  now  approach 
the  early  forms  of  radio  transmitters  of  Marconi  and  Lodge. 
In  making  diagrams  of  radio  telegraphic  circuits,  a  num- 
ber of  simple  conventions  are  used  to  represent  the  various 
devices,  as  illustrated  in  Fig.  10. 


40 


CHAPTER  THREE. 

X. 

THE   MARCONI   1896   TRANSMITTER. 

69.  Marconi,  who  is  at  present  an  officer  in  the  Italian 
Army,  is  an  Italian  but  spent  a  great  many  years  of  his 


Lines  of  Force  about  a 
Hertz  Oscillator 


Lines  of  Force  about  a 
Grounded  Antenna 

Fig.  ii. 

life  in  England  and  did  most  of  his  scientific  work  there. 
His  greatest  contribution  to  the  art  was  the  adoption  of  the 
4  41 


70] 


ELEMENTS   OF  RADIOTELEGRAPHY. 


grounded  antenna,  which  as  noted  in  paragraph  13  is  in  use 
at  the  present  time.  Prior  to  Marconi,  Hertz,  a  German, 
had  employed  ungrounded  waves  for  radio  purposes,  but 

their  range  of  transmission  was 
limited.  Hertzian  waves,  as  they 
are  called,  are  radiated  in  straight 
lines,  similar  to  light,  whereas 
the  grounded  waves  of  Marconi 
travel  with  their  "feet"  on  the 
ground,  as  shown  in  Fig.  1 1 .  They 
are  here  shown  as  leaving  the 
transmitting  antenna.  The  sub- 
ject of  the  propagation  of  waves 
will  be  taken  up  in  a  later  chapter. 


OSi 


Fig.  12.     Marconi 
1896  Transmitter. 


70.  The  Marconi  transmitter  of 
1896  is  shown  in  Fig.  12.  It  was 
the  first  practicable  system  of  ra- 
dio communication.  Inserted  in 
the  antenna  circuit  was  a  spark 
gap,  SiS2,  connected  to  a  source 
of  high  voltage.  An  antenna  has 
capacity  as  may  be  seen  from  an  inspection  of  Fig.  13,  the 
aerial  serving  as  one  coating  of  the  condenser  described  in 
paragraph  23,  and  the  ground  as  the  other.  The  air  sepa- 
rating the  two  serves  as  the  dielectric.  When  this  antenna 
capacity  becomes  fully  charged,  the  resistance  of  the  spark 
gap  is  broken  down  and  the  condenser  discharges  in  an 
oscillatory  fashion  as  set  forth  in  paragraph  26.  Waves 
are  thus  set  up  in  the  antenna  circuit  which  are  radiated. 

71.  In  paragraphs  45  to  49,  the  necessity  for  a  slow  dis- 
charge of  this  antenna,  so  that  the  waves  would  not  die  out 
too  quickly  but  would  be  feebly  damped,  was  set  forth. 
The  Marconi  transmitter  of  1896,  known  as  the  plain  aerial 

42 


THE  MARCONI  1896   TRANSMITTER.        [72 

transmitter,  did  not  meet  this  requirement,  as  may  be  seen 
from  the  following :  Equation  (32)  gave  us  the  formula  for 
damping.  In  it,  we  learned  that  the  greater  the  resistance 
and  the  less  the  inductance  of  a  circuit,  the  higher  would 
be  the  damping.  In  Fig.  12,  it  will  be  observed  that  the 
spark  gap  is  inserted  directly  in  the  antenna  circuit.  This 
spark  gap  has  a  very  high  resistance,  and  as  such,  gives  a 
very  large  value  to  R  in  equation  (32).  As  a  result,  very 
high  damping  of  the  antenna  current  results  with  conse- 
quent broad  tuning  as  noted  in  paragraph  58.  Also,  there 
is  no  inductance  in  series  with  the  antenna  which  would 
tend  to  reduce  the  high  decrement  occasioned  by  the  large 
gap  resistance.  As  a  result,  the  Marconi  plain  aerial  trans- 


I 

r 


Fig-  IS- 

mitter  gave  rise  to  a  very  broadly  tuned  wave — its  decre- 
ment was  very  great,  the  radiated  wave  was  very  highly 
damped — and  the  necessary  factors  for  efficient  radio  com- 
munication were  not  realized. 

72.  However,  this  Marconi  transmitter  had  one  redeem- 
ing feature — it  was  a  single  circuit  transmitter.  That  is  to 
say,  there  was  but  one  oscillating  circuit.  As  such,  it  gave 
rise  to  waves  of  but  one  frequency  or  length,  and  was  thus 
greatly  superior  to  a  later  Marconi  transmitter  which  em- 
ployed two  oscillating  circuits  and^thus  radiated  waves  of 
double  frequency.  When  we  say  that  the  latter  type  of 

43 


73] 


ELEMENTS  OF  RADIOTELEGRAPHY. 


transmitter  radiated  two  waves,  we  do  not  mean  that  there 
were  only  two  waves  in  a  train,  but  that  two  wave  lengths 
were  radiated,  or  two  wave  trains  of  different  frequency. 
The  single  circuit  type  of  transmitter  is  practically  the  only 
type  of  transmitter  in  use  today,  and  had  the  original  Mar- 
coni plain  aerial  transmitter  not  possessed  such  high  damp- 
ing, the  development  of  radio  transmitters  would  have  been 
even  more  rapid. 

XI. 
COUPLED    CIRCUITS. 

73.  If  two  oscillating  circuits  containing  inductance  and 
capacity  be  associated  or  coupled  together,  they  act  very 
differently  than  when  they  are  allowed  to  oscillate  by  them- 
selves. Their  action  may  most  easily  be  explained  by  the 
pendulum  analogy  shown  in  Fig.  14.  Assume  two  pendu- 


Fig.  14. 

lums  of  the  same  length  tied  together  on  one  common 
string.  Let  us  start  pendulum  P,  the  primary  pendulum, 
swinging.  We  have  found  that  a  swinging  pendulum 
gradually  comes  to  rest,  its  oscillations  or  swings  are 
damped  out  slowly,  but  in  this  case,  pendulum  P  is 
very  rapidly  damped  for  in  swinging,  it  gives  up  some 

44 


COUPLED   CIRCUITS.  [74 

of  its  energy  to  pendulum  S,  the  secondary  pendulum, 
since  they  are  both  of  the  same  length  and  thus  have 
the  same  time  period.  As  S  receives  energy  from  P,  it 
starts  swinging,  its  swings  gradually  increasing  in  ampli- 
tude as  more  and  more  energy  is  received  from  P.  Finally 
P  has  given  up  so  much  of  its  motion  to  S  that  it  comes  to 
rest.  S  is  then  vibrating  at  its  maximum,* but  as  it  con- 
tinues to  swing,  it,  in  turn,  gives  energy  back  to  P  so  that 
P  begins  to  swing — feebly  at  first,  but  increasing  each  time 
until  eventually  it  has  received  all  the  energy  from  S  and 
the  latter  stops  swinging.  The  whole  cycle  is  then  again 
gone  thru  many  times — the  ball  of  energy,  so  to  speak,  being 
tossed  back  and  forth  between  the  two  pendulums,  first  one 
having  it,  then  the  other.  Thus,  as  the  swings  of  the  one 
pendulum  are  increasing  in  amplitude,  those  of  the  other 
are  decreasing.  In  each  transfer  of  energy  from  one  pendu- 
lum to  the  other,  a  certain  amount  is  wasted  hi  traversing 
the  length  of  string  between  their  two  points  of  support. 

74.  A  picture  or  oscillograph  of  their  swings  is  repre- 
sented in  Fig.  15.  It  will  be  noted  that  when  the  swings 
of  P  are  the  greatest,  those  of  S  are  the  least,  and  vice 


versa.  This  periodic  rising  and  falling  of  the  amplitude  of 
the  swings  is  termed  "beats,"  and  indicates  that  each  pen- 
dulum is  actually  swinging  at  two  different  frequencies,  for 

45 


75]        ELEMENTS   OF  RADIOTELEGRAPHY. 

beats  are  only  formed  by  the  combination  of  oscillations  or 
swings — whether  mechanical  or  electrical — of  two  fre- 
quencies. Beats  may  be  explained  by  the  following  rather 
crude  analogy.  If  two  horses  be  driven  together  and  they 
are  running  at  different  gaits,  they  will  run  in  step  for  a 
short  length  of  time,  then  the  hoof  beats  of  the  slower  will 
gradually  drop  behind  those  of  the  faster  until  he  is  entirely 
"out  of  step,"  as  we  say.  His  steps  will  continue  to  drop 
farther  and  farther  behind  those  of  the  faster  horse  until 
finally  they  begin  to  coincide  again  with  the  latter's  and 
they  are  once  more  running  in  step.  When  the  two  horses 
are  running  in  step,  the  noise  made  by  their  hoofs  is  the 
loudest  because  they  are  in  unison;  it  is  just  twice  that 
made  when  either  horse  is  running  alone  or  when  they  are 
out  of  step.  This  adding  together  when  they  are  in  step  is 
called  beats.  The  same  phenomenon  is  observed  in  the 
study  of  sound,  for  if  two  notes  are  sounded,  differing 
slightly  from  each  other  in  pitch,  there  will  be  a  periodic 
rising  and  falling  of  the  resultant  note  exactly  correspond- 
ing to  the  analogy  quoted  above.  The  occurrence  of  beats 
is  thus  indicative  of  the  presence  of  two  sets  of  oscillations 
or  vibrations  of  different  frequency. 

75.  Just  as  it  is  possible  to  have  two  coupled  mechanic- 
ally oscillating  circuits,  as  in  the  case  of  the  pendulums  in 
Fig.  14,  so  it  is  possible  to  couple  two  electrical  circuits  to- 
gether as  shown  in  Fig.  16.  (a)  is  called  inductive  coup- 
ling, (b)  is  called  conductive  coupling,  and  (c)  is  called 
capacity  or  static  coupling.  The  primary  circuit  of  each 
pair  is  labelled  P  and  the  secondary  circuit  S.  In  Fig.  16 
(a),  the  transfer  of  energy  from  the  primary  to  the  secon- 
dary circuit  is  as  follows :  If  an  alternating  current,  either 
damped  or  undamped,  be  passed  thru  the  inductance  Li, 
the  rising  and  falling  magnetic  field  set  up  thereby  cuts 

46 


COUPLED   CIRCUITS. 


[76 


the  turns  of  L2  and  by  the  principle  of  induction  set  forth 
in  paragraphs  30  and  33,  an  alternating  E.M.F.  is  induced 
across  the  terminals  of  L2.  This  E.M.F.  causes  an  alter- 
nating current  to  flow  in  the  secondary  circuit.  In  Fig. 
16  (b),  the  coupling  may  be  considered  as  of  double  type, 
electro-magnetic  and  conductive.  The  electro-magnetic  or 
inductive  coupling  operates  on  the  same  principle  as  that 
set  forth  for  Fig.  16  (a),  except  that  LI  and  L2  are  identically 


c,: 


CcO 


(b) 


Fig.  16.     Methods  of  Coupling  Circuits. 

the  same  coil.  The  inductive  action,  however,  is  the  same. 
The  conductive  coupling  is  based  on  conduction  rather  than 
induction  and  obtains  even  tho  the  two  circuits  be  coupled 
by  a  non-inductive  resistance. 

76.  That  property  of  two  associated  circuits  by  virtue  of 
which  energy  is  interchanged  between  them  thru  the  med- 
ium of  electro-magnetic  coupling  is  termed  their  mutual 
inductance.  The  mutual  inductance  is  increased  by  in- 
creasing the  number  of  turns  hi  either  of  the  two  coupled 
inductances,  by  placing  them  closer  to  each  other,  or  by 
introducing  iron  or  other  magnetizable  metals  into  either 
inductance.  Thus  the  mutual  inductance  is  increased  if  the 
inductance  of  either  circuit  be  increased  (providing  that 

47 


77]        ELEMENTS  OF  RADIOTELEGRAPHY. 

the  inductance  is  inductively  coupled  to  the  other  circuit), 
and  if  the  two  inductances  be  placed  so  that  a  maximum 
number  of  lines  of  force  set  up  by  the  one  may  thread  or 
cut  the  other.  Thus  in  Fig.  14,  the  distance  between  the 
two  pendulums  on  their  common  support,  AB,  may  be 
considered  as  their  mutual  inductance.  If  the  coupling 
between  these  two  oscillating  circuits  be  weakened  by  fur- 
ther separating  the  two  pendulums,  the  mutual  inductance 
is  weakened.  As  the  two  pendulums  are  fastened  nearer 
to  each  other  on  the  supporting  string,  their  coupling  and 
mutual  inductance  are  increased. 

77.  In  radiotelegraphy,  where  we  make  frequent  use  of 
coupled  oscillating  circuits,  the  two  circuits  are  always 
tuned  to  resonance  as  in  the  case  of  the  pendulums  of  Fig. 
14,  in  order  that  a  maximum  amount  of  energy  may  be 
transferred  from  one  circuit  to  the  other. 

78.  As  in  the  pendulum  experiment,  when  two  oscillat- 
ing circuits  are  coupled  together,  even  tho  each  of  them  be 
adjusted  independently  to  the  same  wave  length,  they  will 
oscillate  at  two  different  frequencies.     An  oscillograph  of 
their  oscillations  will  be  exactly  similar  to  Fig.  15.     The 
formulae  for  the  two  wave  lengths  set  up  in  two  coupled 
circuits  are  given  below: 


,-  (40) 

X2  =  \V(1  -  0), 

where  Xi  represents  the  length  of  one  of  the  coupling  waves, 
X2  represents  the  other  coupling  wave,  X  represents  the  wave 
length  to  which  both  circuits  were  first  independently 
adjusted,  and  |8,  the  Greek  letter  beta,  is  the  coefficient 
of  coupling  as  defined  by  the  equation 

48 


COUPLED   CIRCUITS.  [78 

(41) 

where  M  represents  the  mutual  inductance  as  defined  in 
paragraph  76,  LI  is  the  inductance  of  one  circuit,  and  Lz  is 
the  inductance  of  the  other.  From  an  examination  of  these 
three  formulae,  the  following  is  observed:  The  difference 
between  the  two  wave  lengths  Xi  and  X2  depends  upon  the 
value  of  j8.  It  will  be  seen  that  as  M  is  made  smaller  by 
weakening  the  coupling,  the  two  waves  will  be  brought 
closer  and  closer  together  since  a  smaller  value  of  M  re- 
sults in  a  diminished  value  of  £.  LI  and  L2  in  the  formulas 
are  the  total  values  of  inductance  in  each  circuit,  not  just 
those  portions  of  the  inductance  in  each  circuit  which  hap- 
pen to  be  inductively  coupled  to  each  other,  as  represented 
by  LI  and  L2  in  Fig.  16.  Thus,  if  additional  inductances 
were  inserted  in  each  circuit  and  were  not  inductively 
coupled  to  each  other,  that  is  to  say,  if  the  lines  of  force 
set  up  by  one  did  not  cut  the  other,  then  while  LiL2  of 
equation  (41)  was  increased,  M  would  remain  the  same. 
An  increase  in  the  denominator  results  in  a  smaller  value 
for  |8,  so  that  if  additional  inductance  be  inserted  in  either 
circuit,  we  reduce  the  coefficient  of  coupling,  and  Xi 
and  X2  are  brought  nearer  together,  approach  more  nearly 
the  value  of  X.  It  will  be  seen  that  altho  each  circuit  be 
tuned  independently  to  one  wave  length,  X,  of  the  two  re- 
sultant wave  lengths  in  both  circuits,  one — Xi — is  longer 
than  X,  and  the  other  — X2 — is  shorter  than  X.  Practically, 
it  is  possible  to  weaken  the  coupling  to  such  an  extent  that 
j3  becomes  equal  to  practically  zero  when  Xi  and  X2  equal 
X,  and  there  is  only  one  wave  length  in  each  circuit.  The- 
oretically, it  is  not  possible  to  ever  bring  the  two  wave 
lengths  together  unless  the  two  circuits  are  separated  to 
such  a  degree  that  /3  becomes  actually  equal  to  0,  which 

49 


79]        ELEMENTS  OF  RADIOTELEGRAPHY. 

means  that  there  is  no  coupling  between  the  two  circuits 
and  no  energy  could  be  transferred  from  the  primary  to 
the  secondary.  Practically,  to  separate  the  two  circuits, 
so  as  to  reduce  the  value  of  ft  and  thus  bring  Xi  and 
X2  together,  involves  such  a  weakening  of  coupling  that 
energy  cannot  be  efficiently  transferred  from  the  primary 
to  the  secondaiy,  for  so  long  as  there  is  sufficient  coupling 
to  transfer  energy  from  the  primary  to  the  secondary  it  is 
also  possible  for  the  secondary  to  return  it  to  the  primary, 
as  with  our  pendulum  experiment,  with  the  result  that 
interaction  or  reaction  takes  place  between  the  two  cir- 
cuits and  we  have  the  production  of  two  waves  of  different 
frequency,  Xi  and  X2.  It  should  be  borne  in  mind  that  Xi 
is  not  the  wave  length  of  one  circuit,  and  X2  that  of  the 
other.  On  the  contrary,  both  circuits  oscillate  at  both 
wave  lengths  or  frequencies. 

79.  Returning  to  our  pendulum  experiment  of  paragraph 
73,  if  it  were  possible  to  cut  loose  our  first  pendulum  after 
it  had  given  up  all  its  energy  to  the  second  one,  the  second 
one  could  then  oscillate  at  its  own  period  and  its  swings 
would  not  be  damped  by  giving  back  energy  to  the  primary 
pendulum.  And  similarly  with  two  coupled  oscillating  cir- 
cuits, if  it  were  possible  to  open  circuit  the  primary  after  all 
its  energy  had  been  given  to  the  secondary,  then  no  energy 
could  be  given  back  from  the  latter  to  the  former,  the 
secondary  could  oscillate  by  itself  at  but  one  wave  length, 
X,  and  would  be  damped  out  only  by  its  own  resistance  and 
capacity  as  per  equation  (32).  It  is  true  that  while  the 
primary  would  be  giving  energy  to  the  secondary  and  they 
were  thus  both  oscillating,  two  wave  lengths  would  be  pres- 
ent in  each  circuit,  but  as  soon  as  the  primary  were  open 
circuited,  the  secondary  would  return  to  vibration  at  its 
own  natural  period,  X.  Of  course,  while  the  secondary  is 

50 


COUPLED   CIRCUITS.  [81 

oscillating,  since  LI  and  L2  of  Fig.  16  (a)  are  still  inductively 
coupled,  the  oscillations  in  L2  will  induce  an  E.M.F.  in 
the  inductance  LI,  but  since  we  have  opened  the  circuit, 
no  current  can  flow  and  hence  no  energy  is  returned  to  the 
primary. 

80.  In  Fig.  16,  the  secondary  circuit  may  represent  the 
antenna  circuit  of  a  radio  transmitter,  since  as  we  have 
noted  before,  the  antenna  circuit  is  simply  an  oscillating 
circuit  composed  of  capacity  and  inductance,  and  the  pri- 
mary circuit  may  be  a  circuit  coupled  thereto  for  the  pur- 
pose of  supplying  energy  to  the  antenna. 

XII. 
LODGE   1898  TRANSMITTER. 

81.  In  addition  to  radiating  waves  of  very  high  damping, 
the  Marconi  1896  transmitter  had  the  disadvantage  of  not 
being  able  to  store  in  the  antenna  sufficient  energy  for 
long  range  transmission.     The  amount  of  power  supplied 
to  the  antenna  or  to  any  circuit  containing  capacity  is  given 
by  the  formula 

^"f,  (42) 

where  P  represents  the  power,  n  the  number  of  charges 
given  the  condenser  per  second,  C  the  capacity  and  E  the 
potential.  In  an  antenna,  the  capacity  is  not  very  large, 
about  0.001  microfarads  for  the  average  ship  antenna,  so 
that  in  order  to  supply  a  large  amount  of  power  to  the  an- 
tenna for  radiation  purposes,  the  potential  must  be  high  or 
the  condenser  frequently  charged.  In  the  Marconi  trans- 
mitter of  Fig.  12,  the  source  of  voltage  was  an  induction 
coil,  which,  at  the  most,  gave  about  200  charges  to  the 

51 


82]         ELEMENTS  OF  RADIOTELEGRAPHY. 


condenser  per  second  since  its  vibrator  speed  was  not  very 
high.  If  we  increase  E  in  the  plain  aerial  transmitter  of 
Fig.  12,  we  must  have  a  long  spark  gap,  and  the  longer  the 
spark  gap,  the  greater  its  resistance  with  consequent  dele- 
terious effect  on  the  damping  of  the  oscillations  as  we  have 
previously  observed.  So  that  the  plain  aerial  transmitter 
does  not  offer  any  satisfactory  method  of  putting  a  large 
amount  of  power  into  the  antenna. 

82.  Sir  Oliver  Lodge,  one  of  the  most  famous  of  the  mod- 
ern English  scientists,  recognized  the  inherent  weaknesses 


Fig.  17.     Lodge  1898  Transmitter. 

of  the  plain  aerial  transmitter  in  its  low  energy  content  and 
its  highly  damped  wave  radiation,  and  to  obviate  these 
difficulties  brought  out  his  1898  transmitter  which  is  shown 
in  Fig.  17.  In  order  to  reduce  the  damping  of  the  antenna 
circuit,  he  removed  the  spark  gap  from  the  antenna  and  in- 

52 


LODGE  1898    TRANSMITTER.  [83 

serted  it  in  a  circuit  coupled  thereto  at  the  position  SiS2. 
(It  is  true  that  his  patent  showed  a  spark  gap  in  the  antenna, 
but  this,  he  stated,  was  to  be  closed  when  the  gaps  SiS2 
were  employed.)  This  was  the  chief  defect  of  the  plain 
aerial  transmitter  as  has  previously  been  discussed.  And 
to  further  reduce  the  damping  in  the  antenna  circuit,  he 
inserted  therein  the  large  variable  inductance  coils,  L, 
which,  as  we  have  previously  seen,  will  tend  to  further  re- 
duce the  logarithmic  decrement.  His  antenna  circuit,  then, 
embraced  two  new  features  which  the  plain  aerial  lacked 
to  make  it  a  successful  transmitter,  namely — low  resistance 
by  removal  of  the  spark  gap,  and  high  inductance  by  the 
insertion  of  coils. 

83.  In  order  to  set  his  antenna  into  vibration,  he  chose 
a  most  novel  scheme,  which  can  best  be  explained  by  mak- 
ing use  of  a  physical  analogy.  We  have  seen  how  a  vibra- 
tory body,  such  as  a  tuning  fork,  may  be  set  into  oscillation 
by  waves  radiated  from  another  tuning  fork  of  identical 
pitch  or  wave  length,  but  it  is  also  possible  to  set  such  a 
body  into  vibration  by  a  different  method.  Consider  that 
we  have  a  piano  with  but  one  string  on  it — middle  C,  and 
that  we  wish  to  set  this  string  into  vibration  without  ac- 
tually striking  or  plucking  it.  From  our  knowledge  of  the 
principle  of  resonance,  we  know  that  we  can  set  1his  C 
string  into  oscillation  by  sounding  the  same  note  on  a  tun- 
ing fork,  by  striking  the  C  string  on  another  piano  in  the 
near  vicinity,  or  by  bowing  the  same  note  on  a  violin  held 
near  the  piano.  But  if  we  should  take  a  sledge  hammer 
and  hit  the  back  of  the  piano  a  heavy  blow,  we  would 
set  the  C  string  into  vibration,  and  yet — and  this  is  sig- 
nificant— we  would  not  have  to  exercise  any  care  to  see  that 
our  exciting  force  was  tuned  to  the  period  of  the  string. 
As  a  matter  of  f act,  the  hammer  blow  has  no  pitch  or  period 

53 


84]         ELEMENTS  OF  RADIOTELEGRAPHY. 

or  wave  length  of  its  own  at  all — it  is  non-periodic  or 
aperiodic,  as  we  learned  these  terms  in  paragraph  1.  Such 
a  means  of  exciting  a  vibratory  circuit  into  oscillation  is 
variously  termed  impulse  excitation^  shock  excitation,  and 
whip-crack  excitation,  and  the  principle  may  be  applied  to 
electrically  as  well  as  physically  vibrating  circuits. 

84.  It  was  this  principle  which  Lodge  employed  in  his 
transmitter.     The  coil  K  shown  in  Fig.  17  serves  no  purpose 
other  than  to  charge  the  condensers  C.    Had  Lodge  con- 
nected his  condensers  directly  across  the  source  of  his 
high  potential,  the  necessity  for  coil  K  would  have  been 
obviated.    His  spark  gaps  Si  and  S2  were,  in  the  language 
of  the  patent,  "polished  and  protected  from  ultra-violet 
light,  so  as  to  supply  the  electric  charge  in  as  sudden  a 
manner  as  possible."     Lodge  also  stated  in  his  patent  that 
the  principle  of  operation  of  his  transmitter  was  to  strike 
the  antenna,  as  he   termed  it,  with  a  "lightning  flash," 
leaving  the  antenna  circuit,  after  it  had  been  so  excited 
into  vibration — "to  oscillate,  free  from  any  disturbance 
due  to  maintained  connection  with   the  source  of    elec- 
tricity, and  therefore  oscillate  longer  and  more  simply  than 
when  supplied  by  wires  in  the  usual  way." 

85.  Just  as  Lodge  desired  low  damping  in  the  antenna 
circuit,  so  he  desired  high  damping  in  the  gap  circuit. 
His  objective  in  that  circuit  was  to  simulate  the  hammer 
blow — to  damp  the  discharge  of  the  condensers  C  so  that 
instead  of  oscillating,  there  would  simply  be  a  single  swing 
of  current.     He  thus  had  direct  current  in  his  gap  circuit; 
of  changing  potential  it  is  true,  but  not  changing  in  direc- 
tion.    He  obtained  this  result  by  complying  with  formula 
(34)  which  states  the  factors  necessary  to  obtain  a  non- 
oscillatory  discharge  of  a  condenser.     In  giving  a  very  high 
value  of  resistance  to  his  spark  gaps,  by  screening  them 

54 


LODGE  1898    TRANSMITTER.  [87 

from  ultra-violet  light  which  would  lower  their  resistance, 
he  was  able  to  bring  this  condition  about. 

86.  Since  there  were  no  oscillations  in  his  gap  circuit, 
one  cannot  speak  of  its  frequency  or  of  its  wave  length. 
The  circuit  did  have  a  certain  time  period  as  given  by  equa- 
tion (36),  as  does  any  circuit  containing  inductance  and 
capacity,  but  since  his  gap  circuit  was  virtually  a  direct 
current  circuit  instead  of  an  oscillating  one,  there  was  no 
necessity  for  tuning  his  gap  circuit  to  the  same  time  period 
as  that  of  the  antenna  circuit  for  he  did  not  have  two  coup- 
led oscillating  circuits  as  set  forth  in  Section  XI.     This 
means  that  for  any  wave  length  which  he  wished  to  radiate 
from  the  antenna,  it  was  only  necessary  to  tune  that  circuit 
— the  gap  circuit  having  any  time  period  at  which  he  chose 
to  set  it.     The  gap  circuit  comprised  the  main  gap,  the 
spark  gaps  Si,  S2,  the  condensers  C  and  the  inductances  L. 
The  antenna  circuit  of  course  consisted  of  the  antenna  4, 
the  inductances  L  and  the  earth  connection  G.     The  two 
circuits  were  thus  conductively  connected  as  per  Fig.  16  (b). 

87.  In  the  section  under  "Coupled  Circuits,"  paragraph 
79,  we  observed  that  if  it  were  possible  to  cut  one  pendu- 
lum loose  from  the  other  after  it  had  given  up  all  its  energy 
to  the  latter,  the  second  one  could  then  proceed  to  "oscil- 
late at  its  own  period  and  its  swings  would  not  be  damped 
by  giving  back  energy  to  the  primary  pendulum."     The 
two  situations  are  seen  to  be  exactly  similar,  and  Lodge, 
altho  he  had  two  circuits  coupled  together,  had  but  one 
oscillating  circuit,   the  antenna  circuit.     His  transmitter 
was  thus  a  single  circuit  type  of  transmitter,  but  unlike  the 
Marconi  plain  aerial  transmitter,  which  was  also  a  single 
circuit  type  of  transmitter,  with  its  high  resistance  and  low 
inductance,  giving  rise  to  highly  damped  waves,  Lodge's 

5  55 


88]         ELEMENTS   OF  RADIOTELEGRAPHY. 

had  low  resistance   and  high  inductance  in  the  antenna 
circuit  and  thus  radiated  a  single  wave  of  feeble  damping. 

88.  Lodge,  in  specifying  that  his  spark  gaps  should  be 
screened  from  ultra-violet  light  was  years  ahead  of  his  time, 
for  in  so  stipulating,  he  had  designed  the  modern  quenched 
spark  gap.  Before  leaving  the  Lodge  transmitter  we  shall  in- 
vestigate the  subject  of  spark  gaps,  which,  in  the  two  types 
of  transmitters  we  have  studied,  plays  so  important  a  part. 


56 


CHAPTER  FOUR. 

XIII. 
THEORY   OF  IONIZATION. 

89.  When  a  substance  cannot  be  divided  or  split  up  by 
mechanical  or  chemical  action  into  any  substance  other 
than  itself,  it  is  said  to  be  an  element.    Thus  zinc  and 
copper  are  elements,  for  no  matter  how  much  either  of 
them  may  be  heated  or  broken  up,  it  cannot  be  so  separated 
or  decomposed  as  to  produce  any  other  substance  than 
itself.     Brass,  on  the  other  hand,  may  be  dissociated  into 
zinc  and  copper.     It  is  an  alloy  of  these  two  metals  and  is 
termed  a  compound.    And  similarly,  the  compound  galena, 
which  is  used  as  a  detector  in  radiotelegraphy,  may  be  re- 
solved into  the  two  elements  lead  and  sulphur.     The  small- 
est part  of  an  element  which  can  exist  is  termed  an  atom. 
The  smallest  part  of  a  compound  which  can  exist  is  called 
a  molecule.     A  molecule  is  thus  seen  to  be  composed  of 
at  least  two  atoms,  and  usually  atoms  of  difference  elements. 
One  molecule  of  water  is  composed  of  two  atoms  of  hydro- 
gen and  one  of  oxygen,  and  representing  hydrogen  by  H, 
and  oxygen  with  O,  a  molecule  of  water  is  written  as  H2O. 

90.  Many   compounds   when  they   are   dissolved   in  a 
liquid  such  as  water  go  through  a  certain  chemical  change. 
Some  of  their  molecules  fall  apart,  or  dissociate,  into  two 
or  more  parts  which  are  termed  ions.    Thus  common  salt, 
a  compound  formed  of  sodium  and  chlorine  and  represented 
by  the  symbol  NaCl,  when  dissolved  in  water  dissociates 
into  the  sodium  ion  and  the  chlorine  ion.     These  ions  are 
free  to  move  about  hi  the  solution  apart  from  each  other, 
and  hence  are  given  the  name  "ion,"  from  the  Greek  word 
meaning  "wanderer." 

57 


91]         ELEMENTS   OF  RADIOTELEGRAPHY. 

91.  The  significant  part  about  an  ion  is  that  it  carries  an 
electrical  charge.     If  an  ion  loses  its  charge  for  any  reason, 
it  becomes  simply  an  atom,  so  that  we  may  define  the  ion 
as  an  atom  with  an  electrical  charge.*     Some  ions  are  posi- 
tively charged,  some  negative,  depending  on  the  element. 
For  instance,  in  salt,  a  compound  of  sodium  and  chlorine 
as  noted  above,  the  sodium  ion  is  positive  and  the  chlorine 
ion  negative.     Thus  for  every  molecule  of  salt  which  is 
dissociated,  two  ions  are  liberated — one  positive  and  the 
other  negative.    The  total  charge  of  the  positive  ions  liber- 
ated by  the  dissociation  of  a  compound  is  always  equal  to 
that  of  the  negative  ions.     Before  a  molecule  of  salt  is  dis- 
sociated by  dissolving  it  in  water,  its  total  electrical  charge 
is  nil,  for  the  positive  charge  of  the  sodium  is  cancelled 
or  annulled  by  the  negative  charge  of  the  chlorine  ion.     But 
when  it  is  broken  up  or  dissociated  in  a  solution,  while  for 
every  positive  ion  liberated   there  is  a  negative  ion  set 
free  of  equal  charge  but  opposite  sign  or  polarity,  and  the 
total  charge  of  the  solution  may  thus  be  considered  as 
zero,  these  positive  and  negative  ions  are  free  to  move 
about  as  they  will  and  very  effectively  demonstrate  their 
electrical  properties. 

92.  If  a  solution  of  common  salt  be  placed  in  a  container, 
as  shown  in  Fig.  18,  into  which  metal  strips  connected  to  a 
source  of  electricity  have  been  inserted  (termed  electrodes), 
these  ions  proceed  to  move  about  very  actively.     We  say, 
that  unlike  charges   of   electricity  attract  each  other  and 
like  charges  repel.     In  Fig.  18,  the  electrode  A  is  positively 
charged  since  it  is  connected  to  the  positive  side  of  the 
battery,  while  B  is  negatively  charged.     The  negative  or 

*  In  some  solutions,  two  or  more  atoms  may  unite,  and  when 
ionized  will  act  as  a  single  ion.  Thus,  in  sulphuric  acid,  H2SO4,  the 
single  atom  of  sulphur  and  four  of  oxygen  act,  when  charged,  as  a 
single  ion,  called  the  sulphion,  and  designated  as  SO^. 

58 


THEORY  OF  IONIZATION. 


[93 


chlorine  ions,  being  attracted  by  the  positive  or  unlike 
charge  on  A,  move  toward  it  and  cluster  around  it,  while 
the  positive  or  sodium  ions  are  attracted  by  B  and  move 
toward  it.  These  ions  in  their  flight  are  naturally  carrying 
their  electrical  charges  with  them,  and  the  motion  or  dis- 
placement of  these  electrical  charges  from  one  side  of 
the  solution  to  the  other  constitutes  the  passage  of  an 
electric  current  thru  it.  Such  a 
solution,  having  the  property  of  con- 
ducting an  electric  current,  is  termed 
an  electrolyte.  (See  paragraph  22.) 
When  the  ions  reach  their  respective 
destinations,  they  give  up  their 
charges  to  the  electrodes  with  which 
they  come  in  contact,  and  become 
atoms  once  more.  To  pass  a  current 
thru  a  solution,  therefore,  it  is  neces- 
sary that  the  molecules  of  the  sub- 
stance dissolved  therein  be  dissociated  into  its  electrically 
charged  ions.  These  ions,  under  the  influence  of  the  elec- 
trical attraction  of  the  electrodes,  are  carried  to  the  elec- 
trodes where  they  give  up  their  charges,  and  in  their  move- 
ment or  flight  have  brought  about  the  passage  of  the  elec- 
tric current. 

XIV. 

SPARK   GAPS. 

93.  The  conduction  of  electricity  thru  a  gas  is  very  sim- 
ilar to  that  in  liquids.  Fig.  19  shows  what 
may  be  considered  to  be  a  molecule  of  a  gas, 
consisting  of  a  positive  charge  surrounded  by 
negative  charges.  As  in  the  molecule  of  a 
substance,  the  total  positive  charge  equals 
the  total  negative  charge,  so  that  the  elec- 
59 


94]         ELEMENTS   OF  RADIOTELEGRAPHY. 

trical  charge  of  the  undissociated  molecule  is  nil.  While 
it  is  only  necessary  to  dissolve  a  substance  in  water  in 
order  to  break  it  up  or  dissociate  it  into  its  respective  ions, 
a  more  elaborate  procedure  is  necessary  in  order  to  disso- 
ciate the  molecules  of  a  gas. 

94.  There  are  everywhere  present  in  the  earth  minute 
particles  of  radio-active  substances,  such  as  radium,  which 
are  constantly  radiating  electrons  or  negative  ions.     In  ad- 
dition, sunlight — with  its  accompanying  ultra-violet  light — 
is  also  another  source  of  negatively  charged  electrons.     So 
that  in  every  gas,  there  are  always  a  few  stray  negative 
ions  moving  about,  coming  from  the  sources  just  noted. 

95.  Fig.  20  shows  a  spark  gap,  consisting  of  the  gap  elec- 
trodes A  and  B,  connected  to  a  source  of  high  potential. 
A  is  positively  charged  and  B  negatively  so.     We  will  as- 
sume that  the  difference  of 
potential    or     electromotive 
force  between  the  two  plates 
is  30,000  volts.     With  these 
highly    charged    electrodes 
placed  in  the   gas,  in  this 
case  —  air    at     atmospheric 

I — \JUUL8JL/ — I  pressure,  the   few  negative 

ions  present  as  noted  in  the 
Fig  2Q  preceding  paragraph,  are  at- 

tracted  by,  and  move  with 

very  great  speed  toward,  the  positive  electrode  A.  In  their 
flight,  these  negative  ions  collide  with  the  undissocia- 
ted molecules  of  air,  breaking  the  latter  up  by  the  me- 
chanical impact  of  the  collision  into  their  constituents, 
positive  and  negative  ions.  These  positive  ions  so  liber- 
ated are  attracted  to  the  negative  electrode  5,  and  the 
negative  ones  join  the  original  electrons  in  moving  toward 

60 


SPARK  GAPS.  [97 

A.  These  ions  in  turn  collide  with  other  molecules,  and 
additional  ions  are  thus  liberated  until,  we  say,  the  gas  is 
ionized.  In  a  chemical  solution,  ionization  takes  place  as 
soon  as  the  substance  is  dissolved — in  a  gas,  however,  it 
takes  the  application  of  an  electrical  or  static  potential  or 
field  to  bring  about  its  ionization.  And,  just  as  we  noted  in 
paragraph  92  in  the  case  of  a  solution,  so  the  movement  of 
these  ions  in  a  gas  to  the  positive  and  negative  electrodes, 
according  to  the  respective  charges  on  the  ions,  constitutes 
the  passage  of  an  electric  current  between  them.  Each 
ion  arriving  at  an  electrode  gives  up  to  the  latter  its  charge. 
The  greater  the  difference  of  potential  between  A  and  B  of 
Fig.  20,  the  greater  the  attraction  exerted  on  the  ions  and 
the  faster  they  will  move,  resulting  in  more  collisions  per 
unit  of  time,  with  a  consequent  greater  number  of  ions  ar- 
riving at  each  electrode  per  second  with  their  electrical 
charges  to  deliver.  And  since  the  electric  current  across 
the  air  gap  consists  of  the  summation  or  addition  of  these 
charges,  we  can  see  that  the  greater  the  potential  the  greater 
will  be  the  current.  The  amount  of  current  is  not  in  accord- 
ance with  Ohm's  Law,  however,  as  we  shall  observe  in  a 
later  consideration  of  vacuum  detectors. 

96.  When  the  electric  field  exceeds  the  dielectric  strength 
of  a  gas,  the  passage  of  the  electric  current  thru  it  is  termed 
the  electric  spark.    The  term  arc  is  also  applied  to  a  certain 
type  of  spark,  which  will  be  discussed  in  a  later  chapter. 

97.  When  a  gas  is  heated,  it,  like  the  heated  poker  of 
paragraph  2,  has  an  increased  value  of  molecular  activity, 
that  is  to  say,  the  molecules  possess  more  energy  and  move 
about  at  greater  velocities.     Thus  for  any  given  difference 
of  potential  across  a  gap,  the  greater  the  temperature  of  the 
gas,  the  more  quickly  will  the  ions  move  to  the  electrodes, 
and,  as  noted  in  paragraph  95,  the  greater  will  be  the  current. 

61 


98]         ELEMENTS   OF  RADIOTELEGRAPHY. 

Since,  with  a  given  potential,  the  current  has  increased, 
the  resistance  must  have  decreased.  The  resistance  of  a 
gap  is  thus  observed  to  be  lower  when  the  gas  between  its 
electrodes  is  heated,  and  conversely,  to  obtain  a  high  re- 
sistance spark  gap,  it  is  essential  that  it  be  efficiently  cooled. 

98.  The  removal  of  the  ions  of  the  gas  between  two  elec- 
trodes, by  any  artifice — in  order  that  its  resistance  may  be 
increased — is  called  deionization  of  the  gap. 

99.  In  addition  to  the  ions  existing  in  a  gas  due  to  the 
presence  of  radio-active  substances  everywhere  resident  in 
the  earth,  those  accompanying  the  ultra-violet  rays  of  sun- 
light, and  those  liberated  by  the  mechanical  fracture  of  a 
molecule,  the  intense  heat  generated  on  the  electrodes 
themselves  by  the  electric  spark,  volatilizes  the  metal  into 
a  metallic  vapor.     This  is  particularly  true  of  soft  sub- 
stances such  as  carbon,  copper  and  zinc.     These  heated 
metallic  ions  tend  to  further  reduce  the  resistance  of  the 
gap. 

100.  In  paragraph  85,  we  observed  that  in  order  to  have 
a  high  value  of  resistance  in  the  gap  circuit  of  his  transmit- 
ter so  as  to  damp  the  current  to  but  a  single  swing  or  half 
cycle,  Lodge  increased  the  resistance  of  his  spark  gaps. 
It  would  obviously  not  serve  to  insert  a  fixed  high  resist- 
ance in  the  gap  circuit  for  this  purpose,  for  that  would  merely 
tend  to  waste  the  energy  which  would  have  to  be  expended 
in  overcoming  it.     What  is  desired  is  a  resistance  which 
will  vary  with  time,  that  is  to  say,  which  will  have  a  low 
resistance  while  the  first  discharge  current  rush  of  the 
condenser  is  occurring,  but  which  then  will  very  speedily 
rise  so  as  to  choke  off  any  further  current.     Going  back  to 
our  pendulum  analogy  it  would  be  equivalent  to  having  some 
liquid  which  would  exert  very  little  resistance  against  the 
pendulum  during  the  first  part  of  its  swing  but  whose  fric- 

62 


SPARK  GAPS.  [102 

tion  would  quickly  increase  as  the  pendulum  neared  the 
vertical  position,  and  as  a  consequence  stop  it  suddenly. 
The  various  methods  by  which  a  spark  gap  may  be  deion- 
ized  so  as  to  obtain  a  high  resistance  have  been  discussed 
by  Zenneck,  a  German  professor  and  author,  and  will  now 
be  considered. 

101.  Cooling. — As  observed  in  paragraph  99,  the  cooling 
of  the  spark  gap  and  the  gases  therein  is  necessary  for  de- 
ionization.     This  is  accomplished  in  the  type  of  gap  shown 
in  Fig.  20  by  directing  an  air  blast  from  some  form  of 
blower  or  fan  directly  on  the  gap.     Besides  dissipating  the 
heat  generated  on  the  electrodes  themselves,  it  also  sup- 
plies fresh,  cool,  undissociated  molecules  of  air  as  well  as 
drives  most  of  the  heated  ions  out  of  the  electric  field.   In  the 
quenched  gap,  which  will  be  taken  up  in  some  detail  later, 
the  air  blast  serves  to  cool  the  electrodes  and  indirectly 
the  gases  contained  between  them,  for  this  type  of  gap  is 
an  enclosed  one  and  the  ions  can  not  be  driven  out  by  a 
blast  as  in  the  open  type  of  gap  of  Fig.  20. 

102.  Diffusion. — Just  as  illuminating  gas  emerging  from 
a  gas  jet  spreads  out  in  all  directions  or  diffuses  into  the 
surrounding  atmosphere,  so  the  ions  present  in  a  spark 
gap,  if  it  be  open,  will  diffuse  into  the  surrounding  air. 
The  more  rapid  the  diffusion,  the  more  rapid  will  be  the 
deionization  of  the  gap,  since,  as  the  conducting  ions  leave 
the  space  between  the  electrodes  to  diffuse  into  the  sur- 
rounding gases,  their  absence  results  in  increased  resist- 
ance.    The  air  blast  noted  in  the  preceding  paragraph  as- 
sists in  diffusion,  since  the  ions  are  actually  blown  or  forced 
out  of  the  gap  region  into  the  air  surrounding  it.     Some 
gases  have  a  higher  rate  of  diffusion  than  others — hydrogen 
greater   than   air,    for    example.     Consequently    we   find 
hydrogen  used  in  the  spark  gaps  of  several  systems,  in  the 

63 


103]       ELEMENTS   OF  RADIOTELEGRAPHY. 

form  of  vaporized  alcohol,  illuminating  gas,  or  other  hydro- 
carbons, for  the  express  purpose  of  increasing  the  rate  of 
diffusion  of  the  gases  contained  therein  and  hence  their 
deionization.  This  is  done  in  the  Poulsen  arc,  to  be  de- 
scribed later,  and  in  several  forms  of  quenched  spark  gaps. 

103.  Absorption. — When  an  ion  comes  near  a  metal, 
even  tho  the  metal  be  uncharged,  an  attractive  force  is 
exerted  between  it  and  the  charged  ion,  with  the  result 
that  the  ion  comes  in  contact  with  it  and  gives  up  its  charge. 
As  soon  as  the  ion  loses  its  charge,  it  is  no  longer  useful 
for  the  conduction  of   the  electric  current  and   deioniza- 
tion has  taken  place.     The  larger  the  surface  of  the  elec- 
trodes is  made,  the  more  chance  there  will  be  of  absorp- 
tion on  their  faces,  and  the  closer  they  are  placed  to  each 
other,  the  more  opportunity  there  will  be  afforded  for  the 
ions  to  give  up  their  charges  to  the  metal  in  such  close 
proximity.     It  will  be  readily  seen  that  with  several  plates 
of  metal  in  series,  increased  absorption  is  effected.     This 
fact  is  responsible  for  the  design  of  the  modern  quenched 
gap  and  for  the  use,  by  Lodge,  of  the  three  spark  gaps  in 
his  transmitter  of  Fig.  17. 

104.  Electric  Field. — We  have  observed  in  the  discus- 
sion of  ionization  of  a  solution,  that  the  difference  of  poten- 
tial or  the  electric  field  between  the  electrodes  causes  the 
ions  to  move  to  the  electrodes  where  they  release  their 
charge,  and,  in  returning  them  to  atoms,  effects  the  de- 
ionization of  the  solution.     The  same  procedure  obtains 
in  spark  gaps.     Obviously,  the  less  the  separation  of  the 
electrodes,  the  more  intense  will  be  the  effect  of  their  re- 
spective electrical  charges  on  the  ions  between  the  disks, 
so  that  as  noted  in  the  preceding  paragraph,  a  minute  dis- 
tance between  the  sparking  surfaces  is  essential  for  effec- 
tive deionization. 

64 


SPARK  GAPS.  [107 

105.  Compressed  Air. — Another  method  for  obtaining 
increased  resistance  of   an  enclosed  gap  is  by  the  use  of 
compressed  air.     When  air,  or  any  gas,  is  under  relatively 
high  pressure,  the  molecules  are  very  closely  crowded  to- 
gether, so  closely,  in  fact,  that  they  are  not  so  free  to  move 
about,  just  as   the   movement   of   persons  in  a  crowded 
street  car  is  hampered.     When  the  molecules  or  their  dis- 
sociated ions  are  not  free  to  move  about,  no  conduction  of 
electricity  can  occur,  for  we  have  noted  in  paragraphs  92 
and  95  that  it  is  the  motion  of  these  ions  which  is  respons- 
ible for  the  passage  of  an  electric  current  thru  a  solution 
or  a  gas.     Compressed  air  at  a  pressure  of  160  pounds  to 
the  square  inch  has,  in  fact,  so  high  a  resistance  as  to  make 
it  a  dielectric  even  to  high  potentials,  and  as  such  was  first 
used  in  a  compressed  air  condenser  designed  by  Reginald 
A.  Fessenden,  an  American  inventor.     Air  under  pressure 
is  sometimes  admitted  into  quenched  gaps  for  this  purpose, 
and  the  author  has  designed  a  spark  gap,*  in  which  the 
high  pressure  of  alcohol  vapor  generated  by  the  heat  of  an 
impulse  spark  gap  is  used  to  obtain  a  high  resistance. 

106.  Magnetic  Field. — Just  as  an  electric  field,  as  noted 
in  paragraph  104,  may  assist  in  the  deionization  of  a  spark 
gap,  so  may  a  magnetic  field,  set  up  by  two  electromagnets 
at  right  angles  to  the  flow  of  ions  across  the  gap,  be  used 
for  this  purpose.     The  action  of  this  type  of  field  will  be 
considered  in  a  later  chapter  on  the  Poulsen  Arc. 

107.  Returning  to  the  Lodge  transmitter,  in  the  stipu- 
lation that  his  gaps  should  be  screened  from  ultra-violet 
light  so  as  to  enhance  their  deionization,  and  in  employing 
a  plurality  of  gaps  so  as  to  assist  in  the  ready  absorption 
at  the  surfaces  of  the  gap  electrodes  of  those  ions  present, 

*  Described  in  the  June,  1916,  issue  of  the  "  Proceedings  "  of  the 
Institute  of  Radio  Engineers. 

65 


108]       ELEMENTS   OF  RADIOTELEGRAPHY. 

Lodge  demonstrated  that  he  was  fully  cognizant  of  the 
essential  features  for  successful  radio  transmission,  i.e., 
the  use  of  a  single  circuit  transmitter  of  low  decrement  for 
the  radiation  of  a  wave  of  single  frequency  and  of  feeble 
damping,  by  the  excitation  of  his  antenna  circuit  from  an 
impulse  circuit  coupled  thereto. 

XV. 
MARCONI   1900   TRANSMITTER. 

108.  Following  the  advent  of  the  Lodge  transmitter  which 
incorporated  the  removal  of  the  spark  gap  from  the  antenna 
circuit  and  the  insertion  of  inductance  therein — both  steps 
looking  to  the  reduction  of  the  high  damping  of  the  antenna 
current  of  the  Marconi  1896  transmitter— Marconi  brought 
out  a  new  type  of  transmitter  in  1900,  shown  in  Fig.  21,  in 
which  S  is  an  open  type  of  spark  gap  similar  to  that  in  Fig. 
20,  C  is  a  condenser,  LI  and  £2  are  the  windings   of   an 
oscillation  transformer  or  antenna  coupler  by  means  of 
whose  mutual  inductance  energy  is  transferred  from  the 
primary  or  gap  circuit  to  the  antenna  or  secondary  circuit, 
L3  is  a  variable  inductance  in  series  with  the  antenna  cir- 
cuit for  the  purpose  of  changing  its  wave  length,  A  is  the 
antenna,  and  G  is  the  ground. 

109.  The  principle  of  operation  of  the  Marconi  coupled 
tuned  circuit  transmitter,  as  this  type  of  transmitter  is 
termed,  is  exactly  similar  to  the  action  of  two  inductively 
coupled  circuits  as  set  forth  in  paragraphs  73  to  80.     The 
condenser   C,  having  become  charged,  discharges  in  an 
oscillatory  fashion  across  the  spark  gap  S  and  thru  the  in- 
ductance LI.     The  alternating  current  passing  thru  this 
coil  induces  an  alternating  potential  across  the  terminals 
of  L2  with  a  consequent  flow  of  energy  in  the  antenna  cir- 

66 


MARCONI  1900    TRANSMITTER. 


[109 


cuit.  There  being  no  means  stipulated,  however,  for  en- 
hancing the  resistance  of  the  spark  gap  S,  the  heated 
metallic  vapor  present  between  its  electrodes  serves  to 
make  the  primary  or  gap  circuit,  S-C-Li,  a  closed  cir- 
cuit, as  Marconi  himself  described  it,  with  the  result  that 
the  E.M.F.  induced  across  the  terminals  of  LI  by  the  oscil- 
lations occurring  in  the  antenna  circuit  thru  Z^  is  sufficient 


Fig.  21.     Marconi  1900  Transmitter. 

to  ionize  the  gap  S,  thus  permitting  the  flow  of  current 
in  the  primary  circuit.  The  see-sawing  or  interchange  of 
energy  back  and  forth  between  the  primary  and  secondary 
circuits,  as  described  hi  Section  XI  on  "  Coupled  Cir- 
cuits," thus  takes  place  with  the  presence  in  both  primary 
and  secondary  circuits  of  oscillations  of  double  frequency. 
The  nature  of  the  waves  radiated  by  this  type  of  trans- 

67 


110]       ELEMENTS   OF  RADIOTELEGRAPHY. 

mitteris  similar  to  that  labelled  "  S"  in  Fig.  15,  instead  of 
to  the  feebly  damped  single  wave  radiated  by  the  Lodge 
transmitter  as  shown  in  Fig.  9. 

110.  Marconi  had  the  correct  principle  in  tuning  his  gap 
and  antenna  circuits  to  resonance  for  the  maximum  trans- 
fer of  energy  from  the  one  circuit  to  the  other,  but  in  stipu- 
lating that  his  primary  circuit  should  act  as  a  "  reservoir  " 
circuit,  from  which  energy  would  be  constantly  fed  into  the 
antenna  to  keep  it  in  vibration,  instead  of  providing  for  the 
detachment  of  the  primary  circuit  from  the  secondary  after 
the  formei's  energy  had  all  been  transferred  to  the  latter 
in  order  that  the  antenna  circuit  might  "  oscillate  free  from 
any  disturbance  due  to  maintained  connection  with  the 
source  of  electricity "  as  did  Lodge,  he  ignored  the  dele- 
terious effect  of  coupled  circuits  in  that  they  give  rise  to 
waves  of  double  frequency.  As  a  result,  while  this  type 
of  transmitter  enjoyed  considerable  popularity  both  in  this 
country  and  abroad  for  many  years,  it  was  practically  legis- 
lated out  of  existence  by  the  Radio  Law  of  1912  which  stip- 
ulated that  in  addition  to  feeble  damping,  the  waves  sent 
out  from  an  antenna  must  be  pure,  that  is  to  say,  they  must 
be  of  single  frequency.  As  a  compromise,  however,  the 
Government  will  approve  a  transmitter  which  radiates  a 
wave  of  double  frequency,  providing  that  the  energy  in 
the  wave  of  least  energy  does  not  exceed  10%  of  that  in 
the  greater.  By  weakening  the  coupling  between  the  pri- 
mary and  secondary  circuits  of  the  Marconi  transmitter  as 
noted  in  Section  XI,  it  is  possible  to  comply  with  this  latter 
provision,  but  the  efficiency  of  transfer  of  energy  between 
the  two  circuits  is  greatly  reduced  by  this  method.  The 
reader  is  urged  to  review  the  section  on  coupled  circuits  in 
order  to  fully  understand  the  principle  of  operation  of  this 
type  of  transmitter. 

68 


THE    QUENCHED   SPARK  GAP.  [113 

111.  In  stipulating  that  his  primary  circuit  was  to  act  as 
a  constant  source  of  energy  for  the  secondary  circuit,  Mar- 
coni made  his  primary  circuit  virtually  a  driving  circuit,  the 
oscillations  in  the  antenna  circuit  being  forced  oscillations, 
as  we  have  previously  defined  the  term,  instead  of  the  free 
oscillations  of  the  Lodge  antenna  circuit. 

112.  Had  Lodge  known  how  to  more  effectively  deionize 
his  spark  gaps,  of  the  advantage  of  which  procedure  he  was 
fully  cognizant,  other  than  the  stipulation  that  they  should 
be  screened  from  the  ionizing  effects  of  ultra-violet  light, 
his  transmitter  would  have  earlier  met  with  the  adoption 
which  it  merited.     Recognizing  the  value  of  Lodge's  inven- 
tion, Prof.  Max  Wien,  Baron  von  Lepel,  and  W.  Peuckert, 
all  German  engineers,  adopted  his  principle  of  removing 
the  gap  circuit  from  the  sphere  of  action  after  the  antenna 
was  oscillating,  and  increased  its  efficiency  in  their  design 
of  the  quenched  spark  gap. 

XVI. 
THE    QUENCHED   SPARK   GAP. 

113.  Fig.  22  shows  a  drawing  of  the  modern  quenched 
spark  gap  as  used,  with  occasional  slight  variation,  in  the 
Navy.     Copper  disks  about  eight  inches  in  diameter  are 
cast  and   lathe-turned  as  shown.     The  discharge  surface 
is  very  often  of  silver,  either  welded  or  plated  to  the  copper. 
Silver  is  the  best  conductor  of  heat  as  well  as  of  electricity, 
and  its  use  enhances  the  cooling  qualities  of  the  gap  as  well 
as  reduces  the  resistance  to  the  spark.     (See  paragraph 
100.)     Rings  of  mica  serve  to  insulate  one  plate  from  the 
other  and  to  make  the  gap  air-tight,  and  since  the  mica  is 
about  0.01  inches  in  thickness,  the  length  of  the  spark  be- 
tween each  pair  of  plates  is  about  one  one-hundredth  of  an 

69 


114]       ELEMENTS   OF  RADIOTELEGRAPHY. 


inch.  The  object  in  having  the  groove  cut  in  the  surface, 
noted  in  Fig.  22,  is  to  restrict  the  sparking  to  the  surface 
provided  for  it  and  to  prevent  sparking  around  the  mica, 
which  would  char  it.  In  order  to  jump  the  length  of  gap 
noted  above,  from  1000  to  1200  volts  are  necessary;  thus 

with  ten  gaps  in  series, 
10,000  volts  or  more 
would  be  required.  In 
paragraphs  101  to  106,  we 
discussed  certain  factors 
contributing  to  the  deion- 
ization  of  a  spark  gap. 
That  the  quenched  spark 
gap  fulfills  these  conditions 
may  readily  be  shown. 


Cooling  Flange 


-\--6roove 


*  Discharge  Surface 


£;-  Mica  Ring 


Fig.  22.     Cross-section  of 
Quenched  Spark  Gap. 


(a)  Cooling.  —  Due  to 
the  large  metallic  surface,  the  use  of  silver  or  copper,  the 
external  flanges,  the  use  of  several  gaps  in  series — thus 
increasing  the  internal  and  external  metallic  area,  and  the 
application  of  an  external  air  blast,  the  gap  is  very  effic- 
iently cooled. 

(b)  Diffusion. — In  those  gaps  employing  hydrocarbon  va- 
pors, diffusion  plays  an  important  part.     Scheller  and  Peu- 
kert  of  Germany  and  the  Japanese  Department  of  Commu- 
nications make  use  of  vaporized  alcohol  in  quenched  gaps 
to  enhance  the  diffusion  of  the  ions. 

(c)  Absorption  and  Electric  Field. — Both  of  these  factors 
are  made  use  of  in  the  close  separation  of  the  plates. 


XVII. 

THE  TELEFUNKEN  TRANSMITTER. 

114.  Fig.  23  shows   the   Telefunken  transmitter  which 
has  been  employed  in  many  installations  of  the  Navy.     It 

70 


THE    TELEFUNKEN  TRANSMITTER.        [115 


will  be  noted  that  the  coupling  between  the  antenna  and 
gap  circuits  is  conductive,  corresponding  to  Fig.  16  (b). 


Fig.  23.     Telefunken  Transmitter. 

The  quenched  gap  is  used  exclusively  in  this  type  of  appar- 
atus. 

115.  Fig.  24  shows  the  nature  of  the  oscillations  in  the 
gap  and  antenna  circuits  of  the  Telefunken  transmitter. 


Antenna 


Fig.  24. 
71 


116]       ELEMENTS   OF  RADIOTELEGRAPHY. 

It  will  be  noted  that  while  the  use  of  the  quenched  gap,  by 
its  rapid  deionization  and  hence  speedy  return  to  its  original 
high  resistance,  goes  back  to  the  principle  of  Lodge  in 
exciting  the  antenna  to  vibration  and  then  leaving  it  free 
to  oscillate  by  itself,  the  Lodge  gap  circuit  had  but  one  half 
swing  of  current  in  it  while  the  Telefunken  has  on  the 
average  from  three  to  possibly  eight  or  more.  Neverthe- 
less, the  Telefunken  transmitter  has  all  the  advantages 
of  the  Lodge  in  that  but  a  single  wave  of  feeble  damp- 
ing is  radiated,  and  in  that  free  oscillations  occur  in  the 
antenna. 

116.  Since  the  gap  circuit  of  this  transmitter  is  opened 
the  major  part  of  the  time,  leaving  the  antenna  circuit  only 
in  operation,  this  type  of  apparatus  may  be  considered  a 
single  circuit  transmitter,  differing  from  the  Lodge  trans- 
mitter only  in  degree  of  quenching  in  the  gap  circuit.  How- 
ever, since  there  are  oscillations  in  the  gap  circuit,  even  if 
but  for  a  short  length  of  time,  during  that  portion  of  the 
time,  the  transmitter  is  similar  to  the  coupled  tuned  circuit 
type,  and  hence  for  maximum  transfer  of  energy  between 
the  two  circuits  requires  that«the  two  circuits  be  very  accur- 
ately tuned  with  respect  to  each  other.  The  Telefunken 
Company  states  that  for  best  quenching,  exact  resonance 
between  the  two  circuits  should  not  obtain,  but  rather— a 
detuning  of  about  4.0%;  that  is  to  say,  with  the  gap  circuit 
adjusted  to  600  meters,  the  antenna  circuit  should  be  tuned 
to  within  24  meters  of  that  wave  length.  However, 
whether  resonance  is  employed  or  a  4%  detuning,  the  fact 
remains  that  for  maximum  antenna  current,  a  very  nice 
and  critical  relation  must  obtain  between  these  two  circuits 
which  must  be  preserved  no  matter  to  what  wave  length 
the  antenna  circuit  is  tuned.  With  the  Lodge  transmitter, 
on  the  other  hand,  as  noted  in  paragraph  86,  no  such 

72 


THE   TELEFUNKEN  TRANSMITTER.        [117 

critical  relation  between  the  gap  and  antenna  circuit  exists 
— it  being  merely  necessary  to  vary  the  antenna  circuit 
wave  length  only,  in  order  that  waves  of  different  length 
may  be  radiated. 

117.  Having  studied  the  principles  of  the  various  forms 
of  damped  wave  transmitters,  the  next  chapter  will  be  de- 
voted to  a  consideration  of  their  practical  or  commercial 
construction. 


73 


CHAPTER  FIVE. 

XVIII. 
THE   FOUR  RADIO   TRANSMITTER   CIRCUITS. 

118.  In  Figs.  21  and  23,  that  part  of  the  transmitter  from 
the  source  of  current  supply  thru  the  piimary  of  the  step-up 
transformer — the  coil  on  the  extreme  left — is  termed  the 
primary  circuit.  It  is  the  circuit  of  low  potential  and  low 
frequency  current.  The  secondary  circuit  is  the  high  po- 
tential and  low  frequency  circuit.  It  comprises  the  sec- 
ondary coil  of  the  step-up  transformer — the  next  coil  to  the 
right — and  the  condenser  C.  (It  should  be  borne  in  mind 
that  the  positions  of  the  spark  gap  and  the  condenser  may 
be  interchanged  without  any  substantial  difference  in  the 
operation  of  the  transmitter.  If  this  be  done,  the  conden- 
ser C  is  then  charged  by  the  transformer  thru  the  induc- 
tance LI,  but  as  this  coil  has  very  little  reactance  to  low 
frequency  current,  the  reduction  in  charging  current  is  in- 
appreciable.) The  gap  circuit  is  that  circuit  which  makes 
use  of  the  condenser  discharge  to  produce  oscillations  in 
the  antenna.  It  comprises  the  condenser  C,  the  spark 
gap,  and  the  inductance  LI.  (The  use  of  the  term  "closed 
oscillating"  circuit  as  applied  to  this  circuit  is  inaccurate 
and  the  teim  itself  is  a  misnomer  except  for  the  Marconi 
1900  tuned  coupled  circuit  transmitter.  In  the  modern 
transmitters  employing  high  resistance  gaps,  the  gap  cir- 
cuit is  an  open  circuit  the  major  portion  of  the  time,  as  we 
have  noted  in  paragraph  116,  and  so  far  as  being  an  oscil- 
lating circuit,  every  effort  is  made  to  increase  the  gap  re- 
sistance so  as  to  keep  it  from  oscillating.)  The  antenna 

74 


TRANSMITTING  KEYS.  [121 

circuit,  as  noted  in  paragraph  13,  comprises  the  antenna, 
such  inductances  as  are  used  to  vary  the  wave  length  of 
the  circuit  and  to  induce  energy  from  the  gap  circuit  there- 
in, and  the  ground  connection. 

119.  This  chapter  will  be  given  to  a  discussion  of  the 
various  pieces  of  apparatus  comprising  the  four  circuits 
named  above. 

XIX. 
TRANSMITTING  KEYS. 

120.  In  order  that  signals  may  be  sent  from  a  transmit- 
ting station,  some  method  must  be  supplied  to  regulate  the 
flow  of  current  in  the  transmitter  so  as  to  form  the  dots  and 
dashes  of  the  telegraphic  code.     This  is  done  by  means  of 
the  key,  which  is  a  conveniently  operated  switch  by  which 
the  current  in  the  transmitting  circuits  is  established  and 
interrupted. 

121.  Fig.  25  shows  a  simple  key,  similar  to  the  key  em- 
ployed in  Morse  telegraphy.     The  current  is  passed  into 
the  movable  lever  E  thru  the  trunnion  or  bearing  support 


Fig.  25.     Transmitting  Key. 

C.  As  the  resistance  of  this  bearing  is  often  quite  high, 
it  is  customary — if  the  key  is  to  carry  fairly  heavy  currents 
— to  place  a  copper  spring  F  in  the  position  shown,  to  con- 
duct the  current  from  C  to  the  lever  without  having  to  pass 
thru  the  bearing  of  the  trunnion.  The  lever  carries  a  con- 
tact A  which  engages  with  a  lower  contact  B  when  the  key 

75 


122]       ELEMENTS   OF  RADIOTELEGRAPHY. 

is  depressed,  thus  closing  the  circuit.  A  rubber  knob  D  is 
mounted  on  the  lever  to  insulate  the  operator's  fingers 
from  the  potential  on  the  key.  In  hand  keys,  it  is  custom- 
ary to  make  the  contacts  of  platinum,  or  some  alloy  thereof, 
for  the  flash  or  spark  at  the  contacts  when  the  circuit  is 
broken  will  volatilize  a  softer  metal  with  consequent  burn- 
ing and  sticking  of  the  contacts. 

122.  The  hand  key  described  above  works  quite  satis- 
factorily on  currents  up  to  10  or  15  amperes,  but  when  it  is 
desired  to  use  larger  currents,  some  other  type  of  key  must 
be  used  to  avoid  the  arcing  and  flashing  at  the  contacts, 
which  not  only  materially  damages  the  contacts  but  causes 
them  to  fuse  and  stick — thereby  spoiling  the  character  of 
the  dots  and  dashes.     A  common  type  of  key  for  high 
power  is  the  magnetic  key.    This  is  similar  to  the  relay 
used  in  Morse  telegraphy,  consisting  of  a  lever  or  armature 
— carrying  a  large  contact — actuated   by  an   electromag- 
net.    The  current  in  the  electromagnet  is  direct  current 
of  not  more  than  one  or  two  amperes  and  is  controlled  by 
a  light  telegraph  key.     The  large  contacts  on  the  magnetic 
key  serve  to  establish  and  interrupt  the  heavy  alternating 
current  for  the  radio  transmitter.     The  contacts  on  the  hand 
key  may  be  set  close  together  to  permit  of  rapid  trans- 
mitting, while  the  contacts  on  the  magnetic  key  which  it 
controls  may  be  widely  separated  to  break  the  arc  or  flash 
occurring  between  them. 

123.  When  high  potential  currents  are  to  be  interrupted 
by  a  key,  the  contacts  are  often  immersed  in  oil  which  mate- 
rially reduces  the  sparking.     (Since  there  is  no  air  present 
in  the  oil — combustion  cannot  take  place.)     The  Marconi 
Company  in  its  high  power  installations  mounts  its  magnetic 
keys  in  the  secondary  circuit  where  the  current  is  small. 
This  does  away  with  excessive  heating  and  pitting  of  the 

76 


TRANSMITTING  KEYS. 


[124 


contacts,  but  since  the  voltage  in  this  circuit  is  high,  as 
noted  in  paragraph  118,  a  long  arc  occurs.  This  is  readily 
blown  out  by  an  air  blast  directed  at  the  contacts  from  a 
blower  (see  paragraph  101),  and  the  key  is  consequently 
a  very  efficient  one. 

124.  An  ingenious  key  for  the  elimination  of  sparking, 
used  by  both  the  Marconi  and  Telefunken  Companies,  is 


Fig.  26.     Sparkless  Key. 

shown  in  Fig.  26.  When  the  key  is  depressed,  the  circuit 
is  closed  by  the  contacts  A  and  B  and  an  alternating  cur- 
rent passes  thru  the  magnet  D  which  exerts  an  attraction 
on  the  iron  strip  C,  fastened  to  the  lever.  When  pressure 
is  removed  from  the  key,  the  magnet  does  not  release  the 
lever  until  the  alternating  current  cycle  has  practically 
reached  the  point  of  zero  current.  The  spring  E  then 
pulls  up  the  lever  at  an  instant  when  practically  no  current 
is  flowing  in  the  circuit,  and  consequently  there  is  no 
sparking.  With  60  cycle  current,  since  there  are  120  zero 
points  per  second,  the  most  that  this  type  of  key  could  lag 
behind  the  operator  would  be  one  one-hundred-twentieth 
part  of  a  second,  so  that  in  operating  it,  one  is  not  con- 
scious of  any  lagging  effect.  It  merely  insures  the  break- 
ing of  the  circuit  when  the  alternating  current  is  practically 
at  the  zero  point  of  the  cycle.  (See  Fig.  5.)  Marconi  has 
also  successfully  adapted  this  principle  to  the  magnetic  key. 

77 


125]       ELEMENTS   OF  RADIOTELEGRAPHY. 

125.  Several  automatic  types  of  transmitters  are  in  use. 
These  utilize  paper  tape  which  is  punched  by  a  perforator 
with  a  standard  typewriter  keyboard.  The  tape  is  then  fed 
thru  a  special  transmitter,  the  contacts  of  which  in  passing 
thru  the  holes  in  the  tape  complete  the  circuit.  Fig.  27 
shows  the  letter  R — dot  dash  dot — as  punched  on  tape  by 
two  types  of  perforators.  In  the  strip  to  the  left,  the  dash 
is  made  considerably  longer  than  the  dot,  corresponding  to 
the  actual  hand  transmission  of  these  characters.  In  the 


s    ° 

\        o 

o 
o 

o 

o 

\ 

Fig.  27.     Automatic  Transmitter  Tapes. 

strip  to  the  right,  punched  by  the  Kleinschmidt  perforator,  a 
dot  is  made  by  two  holes  on  the  same  vertical  line  and  a 
dash  by  two  holes  which  are  staggered  or  on  an  oblique  line. 
With  the  automatic  transmitter,  very  high  speeds  can  be 
obtained,  higher — in  fact — than  it  is  possible  to  receive 
except  by  mechanical  means.  Absolute  accuracy  of  trans- 
mission may  be  obtained  by  this  method,  and  in  the  case 
of  dispatches  which  are  to  be  transmitted  more  than  once, 
such  as  press  items  or  other  broadcast  information,  the 
message  may  be  punched  once  on  the  type  and  transmitted 
by  the  one  strip  of  tape  as  many  times  as  desired. 

126.  While,  as  noted  in  paragraph  123,  the  transmitting 
key  may  be  placed  in  the  secondary  circuit  of  a  transmitter, 
it  is  commonly  placed  in  the  primary  circuit  where  the  po- 
tential is  lower,  and  where,  in  the  case  of  the  hand  key, 
there  will  be  less  danger  to  the  operator. 

127.  Certain  types  of  keys  peculiar  to  the  arc  transmitter 
will  be  discussed  in  a  later  chapter. 


78 


TRANSFORMERS.  [129 

XX. 

TRANSFORMERS. 

128.  In  equation  (42)  and  paragraph  81,  it  was  observed 
that  the  amount  of  power  which  could  be  stored  in  a  con- 
denser was  proportional  to  the  capacity  and  the  square  of 
the  potential.     The  size  of  the  condenser  or  its  capacity  is 
limited  by  the  wave  length  to  which  the  gap  circuit  is  to  be 
adjusted,  since  this  wave  length  is  dependent  upon  the 
inductance  and  capacity  of  the  circuit.     A  certain  number 
of  turns  of  inductance  is  necessary  in  order  that  energy 
may  efficiently  be  transferred  from  the  gap  circuit  to  the 
antenna  circuit  by  the  mutual  inductance  of  the  oscillation 
transformer  or  antenna  coupler.     This  requisite  amount  of 
inductance  limits  us  to  a  fixed  maximum  value  of  capacity 
in  order  that  the  wave  length  of  the  circuit  may  not  exceed 
the  desired  amount.     In  a  transmitter  adjusted  to  600 
meters  with  four  turns  of  inductance  hi  the  gap  circuit,  the 
capacity  averages  from  0.009  to  0.012  microfarads.     With 
this  comparatively  low  value  of  capacity,  it  becomes  neces- 
sary to  use  a  fairly  high  potential  hi  order  that  any  consid- 
erable amount  of  power  may  be  used.     Since  it  is  extremely 
difficult  to  manufacture  alternators  of  high  potential,  it  is 
customary  to  employ  alternating  current  generators  of  low 
potential,  1 10  to  500  volts,  and  to  step 

up  the  potential  to  the  desired  amount, 
10,000  volts  or  higher.  This  is  done 
thru  the  medium  of  the  transformer. 

129.  A   transformer   consists   of   two 

coils  of  wire,  insulated  from  each  other,         Fig   2g     open- 
wound  over  a  common  core  of  iron,  from      Core  Transformer, 
which  they  are  also  insulated.     A  sche- 
matic diagram  of  a  transformer  is  shown  in  Fig.  28.    Over 
the  iron  core,  and  carefully  insulated  therefrom,  is  wound 

79 


130]       ELEMENTS   OF  RADIOTELEGRAPHY. 

the  primary  coil  P  of  comparatively  few  turns  of  heavy,  in- 
sulated wire.  Over  the  primary  is  wound  the  secondary 
coil  S  of  a  great  many  turns  of  fine  wire  which  are  equally 
well  insulated.  The  operation  of  the  transformer  is  as 
follows.  When  an  alternating  current  is  caused  to  flow 
thru  P,  the  rising  and  collapsing  lines  of  force  within  the 
iron  core  induce  an  alternating  potential  across  the  ter- 
minals of  the  secondary  as  we  have  previously  observed  in 
paragraph  75. 

130.  In  paragraph  30,  equation  (13),  it  was  observed  that 
the  potential  generated  by  electro-magnetic  means,  as  in 
the  case  of  the  transformer,  is  proportional  to  the  number 
of  turns  of  wire,  and  to  the  number  of  lines  of  force  cut  per 
second.     To  increase  the  secondary  voltage  of  a  radio  trans- 
former, then,  two  methods  may  be  used.     The  number  of 
turns  of  the  secondary,  or  the  number  of  lines  of  force  in 
the  iron  core  produced  per  second  by  the  primary  may  be 
increased.    Since  the  number  of  lines  of  force  is  proportional 
to  the  current  flowing  in  the  primary,  as  may  be  shown ;  to 
increase  the  primary  current  will  result  in  increased  poten- 
tial across  the  secondary  terminals.     To  increase  the  num- 
ber of  lines  per  second ,  the  frequency  of  the  alternating 
current  may  be  increased. 

131.  The  potential  of  the  secondary  and  that  of  the  pri- 
mary bear  a  certain  fixed  relation  to  each  other,  and  is  ex- 
pressed as  follows : 

ES:EP=TS:  Tp,  (43) 

where  Es  is  the  secondary  voltage,  Ep  is  the  primary  volt- 
age, Ts  is  the  number  of  turns  of  wire  in  the  secondary  and 
Tp  is  the  number  of  turns  in  the  primary.  Thus  we  say, 
the  potential  of  the  secondary  is  to  the  potential  of  the  pri- 
mary as  the  number  of  turns  in  the  secondary  is  to  the  num- 
ber of  turns  in  the  primary. 

80 


TRANSFORMERS.  [133 

132.  Thus,  if  there  are  100  turns  in  the  primary  and 
10,000  turns  in  the  secondary,  and  a  potential  of  200  volts 
be  applied  across  the  terminals  of  the  primary,  there  will 
be  a  potential  of  20,000  volts  across  the  secondary.     It 
should  be  borne  in  mind,  however,  that  while  the  potential 
may  be  increased  one  hundred  times  as  in  the  example 
given  above,  the  power  remains  the  same.     (As  a  matter 
of  fact,  there  is  a  slight  loss  of  power  due  to  losses  in  the 
transformer.)     Since  the  power  is  proportional  to  the  prod- 
uct of  the  potential  and  the  current,  for  any  increase  in 
potential  there  must  be  a  corresponding  decrease  in  cur- 
rent.    Thus,  while  the  potentials  of  the  primary  and  sec- 
ondary are  proportional  to  the  number  of  turns  in  each, 
the  currents  in  the  primary  and  secondary  are  inversely 
proportional  to  their  respective  number  of  turns.     This  is 
explanatory  of  the  fact  stated  in  paragraph  123  that  the 
current  in  the  secondary  circuit  is  small,  and  hence  is  eas- 
ily interrupted  by  a  key  designed  for  high  potentials. 

133.  Transformers  are  divided  into  two  classes — open 
and  closed  core.     An  open  core  transformer  is  shown  in 
Fig.  28.     In  this  type,  as  set 

forth  in  paragraph  27,  the 
lines  of  magnetic  force,  or 
the  flux  as  it  is  termed,  run 
thru  the  iron  core  from  end 
to  end  and  back  thru  the 
surrounding  air.  A  closed 
core  transformer  is  shown  Fig.  29.  Closed-Core  Transformer, 
in  Fig.  29.  In  this  type,  the 

flux  has  a  complete  or  closed  iron  path  in  which  to  flow. 
The  reluctance  of  this  path,  that  is  to  say,  the  magnetic 
resistance  offered  to  the  lines  of  force,  is  much  less  than 
in  the  open  core  transformer  for  the  lines  of  force  have 

81 


134]       ELEMENTS   OF  RADIOTELEGRAPHY. 

only  iron  to  traverse   and  are  not  forced  to  travel  partly 
thru  air. 

134.  When  magnetic  flux  rises  or  falls  within  an  iron 
core  due  to  alternating  current  passed  thru  the  primary 
coil,   currents   are   generated   in  the   core   itself  as  well 
as    in   the    secondary.     These   induced   currents    are    of 
no  useful  value  and  represent  a  loss  in  energy;  this  loss 
manifesting  itself  in  the  heating  of  the  iron.     Such  cur- 
rents are  termed  eddy  currents.     They  are  reduced  to  a 
minimum  by  making  up  the  core  of  the  transformer  of  thin 
sheets  of  iron,  called  laminations,  each  sheet  being  elec- 
trically but  not  magnetically  insulated  from  its  adjoining 
one  by  the  resistance  of  its  natural  oxide  or  by  a  thin  coat- 
ing of  shellac,  varnish  or  lacquer.     Since  these  eddy  cur- 
rents tend  to  flow  in  a  direction  at  right  angles  to  the 
length  or  axis  of  the  core,  the  high  resistance  between  the 
laminations  very  effectually  limits  their  magnitude.     Since 
eddy  currents  are  induced  currents,  the  potential  of  which 
is  proportional  to  the  frequency  of  the  alternating  current 
producing  them,  the  losses  in  a  transformer  from  eddy  cur- 
rents increase  with  an  increase  of  frequency. 

135.  All  magnetizable  substances  exhibit  a  peculiar  char- 
acteristic of  retaining  to  some  extent  the  magnetism  set  up 
therein  by  an  electric  current.     In  the  cases  of  some  sub- 
stances such  as  steel  and  wrought  iron,  it  actually  becomes 
necessary  to  pass  a  current  of  electricity  in  the  opposite 
direction  thru  the  primary  in  order  to  demagnetize  the 
core.     It  is  of  course  desired  that  the  flux  within  the  core 
rise  and  fall  as  completely  and  as  quickly  as  the  rising  and 
falling  alternating  current  in  the  primary  in  order  that  the 
induced  potential  in  the  secondary  may  be  at  a  maximum. 
In  a  magnetizable  substance  this  property  of  the  magnetic 
lag  of  the  flux  behind  the  magnetizing  force,  which  in  this 

82 


TRANSFORMERS.  [136 

case  is  the  alternating  current  in  the  primary,  is  termed  its 
hysteresis.  It  is  desired,  as  noted  above,  that  this  hys- 
teresis be  kept  at  a  minimum,  for  the  energy  which  is  re- 
quired to  demagnetize  a  core  after  it  is  once  magnetized, 
and  the  alternating  current  has  fallen  to  zero,  represents  a 
loss  or  waste  similar  to  that  lost  thru  eddy  currents.  The 
hysteresis  of  soft  or  annealed  iron  is  less  than  that  of  any 
other  magnetizable  substance  and  accordingly  it  is  used 
for  the  core  of  an  inductance  coil  thru  which  a  low  fre- 
quency alternating  current  is  to  be  passed. 

136.  While  the  potential  across  the  terminals  of  a  trans- 
former is  that  given  by  the  relation  expressed  in  equation 
(43),  a  different  condition  obtains  when  the  secondary  of  a 
transformer  is  used  to  charge  a  condenser  as  in  Figs.  21 
and  23.  We  have  observed  that  in  an  alternating  current 
circuit  containing  both  capacity  and  inductance,  the  im- 
pedance is  the  least  and  the  current  the  greatest  when  the 
capacity  reactance  is  equal  to  the  inductive  reactance,  for 
in  this  case  the  flow  of  current  is  hindered  only  by  the  re- 
sistance of  the  circuit,  the  resultant  reactance  being  nil. 
See  equation  (20).  The  current  in  the  secondary  cir- 
cuit of  a  transmitter  is  the  greatest,  therefore,  when  the 
condenser  which  the  secondary  is  charging  is  in  resonance 
with  the  charging  circuit.  (It  is  not  strictly  correct  to  say 
that  the  condenser  reactance  is  equal  to  the  secondary  re- 
actance, for  the  latter  is  affected  by  that  of  the  primary 
and  additional  reactance  in  the  primary  circuit.  If,  how- 
ever, we  understand  the  expression  effective  reactance 
of  the  secondary  to  mean  the  resultant  reactance  of  the 
coupled  primary  and  secondary  circuit  inductances,  it  is 
correct  to  say  that  the  reactance  of  the  condenser  should 
equal  the  effective  secondary  reactance  for  resonance.) 
From  Qhm's  Law,  equation  (1),  it  follows  that 

83 


137]       ELEMENTS    OF  RADIOTELEGRAPHY. 

E  =  RI,  (44) 

or  that  the  voltage  or  difference  of  potential  across  a  re- 
sistance is  equal  to  the  resistance  times  the  current  pass- 
ing thru  it.  Similarly,  the  voltage  across  an  inductance 
whose  reactance  is  given  by  equation  (15)  as 

XL  =  2wfL 
is  given  by  the  expression 

EL  =  (27T/L)/,  (45) 

or  the  reactance  times  the  current,  and  the  voltage  across 
a  condenser  whose  reactance  from  equation  (16)  is 


is  expressed  as 


or  the  capacity  reactance  times  the  current.  The  reactance 
of  the  secondary  coil  of  a  transformer  is  very  high,  so  that 
when  the  condenser  which  it  is  charging  is  so  adjusted  as 
to  put  the  secondary  circuit  in  a  state  of  resonance,  the  in- 
creased current  in  the  secondary  circuit  causes  the  potential 
across  the  condenser,  as  given  by  equation  (46),  to  be 
very  much  higher  than  that  which  would  otherwise  exist 
across  it. 

137.  This  condition  of  resonance  may  be  experimentally 
or  empirically  determined  by  inserting  a  milli-ammeter,  an 
instrument  for  the  measurement  of  very  small  currents,  in 
the  secondary  circuit,  as  shown  in  Fig.  30,  in  which  A  rep-. 

84 


TRANSFORMERS. 


[139 


resents  the  milli-ammeter.  The  capacity  of  the  condenser 
is  varied  until  the  greatest  reading  is  obtained  on  the  in- 
strument. This  maximum  current  value  is  indicative  of 
the  state  of  resonance  noted  in  paragraph  136.  The 
ammeter  is  then  removed  from  the  circuit,  and  the  gap 
circuit  coupled  thereto,  as  shown  in  Figs.  21  and  23.  In- 
stead of  varying  the  condenser  capacity  so  as  to  balance 


Fig.  30. 

its  capacity  reactance  with  the  effective  reactance  of  the 
secondary,  the  latter  reactance  may  be  varied  by  a  regu- 
lation of  additional  reactance  in  series  in  the  primary  cir- 
cuit or  by  variation  of  the  alternator  frequency. 

138.  We  have  observed  that  for  the  generation  of  poten- 
tial by  electro-magnetic  means  it  is  necessary  that  there  be 
a  rising  and  falling  magnetic  field.     A  transformer  cannot 
be  used,  therefore,  for  stepping  up  the  potential  of  direct 
current  since  the  magnetic  field  in  its  core  will  be  constant 
instead  of  periodically  rising  and  falling. 

139.  In  order  that  the  potential  of  direct  current  may  be 
stepped  up  by  electro-magnetic  means,  the  induction  coil 
is  used.     This  is  an  instrument  which  incorporates  a  de- 
vice for  making  and  breaking  the  primary  circuit,  so  as  to 
cause  a  rising  and  falling  magnetic  field  in  the  core  of  the 
ordinary  open-core  transformer  previously  described.    A 
diagram  is  shown  in  Fig.  31.     An  armature  carrying  a  yoke 
of  iron,  £,  is  mounted  so  as  to  be  attracted  by  the  magnetism 

7  85 


140]       ELEMENTS   OF  RADIOTELEGRAPHY. 

of  the  core  of  the  induction  coil.  Current  is  passed  into 
the  primary  thru  the  contacts  BC.  A  spring  D  serves  to 
keep  the  armature  E  pulled  back  against  the  contact  C  so 
as  to  close  the  circuit.  In  the  figure,  the  armature  is  shown 
in  the  half  way  position.  When  the  current  flows  in  the 
primary  the  magnetic  flux  set  up  therein  attracts  the  arma- 


Vibrator 


Fig.  31.     Induction  Coil. 

ture  toward  the  core.  As  the  armature  moves  over,  the 
circuit  is  broken  and  the  magnetic  attraction  of  the  core  on 
the  armature  no  longer  exists.  The  spring  D  pulls  the 
armature  back,  the  contacts  BC  are  again  closed  and  the 
whole  cycle  repeats  itself.  This  is  exactly  the  same  action 
which  occurs  in  the  ordinary  electric  bell  or  buzzer.  The 
direct  current  thus  established  and  interrupted  is  not  an 
alternating  current,  for  the  current  does  not  reverse  in  the 
circuit,  altho  it  is  of  intermittent  strength.  It  is  called, 
therefore,  pulsating  direct  current.  Since  the  current  in 
the  primary  is  rising  and  falling,  an  alternating  potential 
is  produced  across  the  terminals  of  the  secondary  as  in  the 
case  of  a  transformer. 

140.  A  diagram  of  the  pulsating  current  in  the  primary 
of  an  induction  coil  is  shown  in  Fig.  32,  which  should  be 
compared  with  the  alternating  current  shown  in  Fig.  5. 
From  A  to  5,  the  current  in  the  primary  circuit  is  building 
up  to  the  strength  represented  at  B.  At  #,  the  vibrator 

86 


TRANSFORMERS. 


[141 


has  been  drawn  over  toward  the  core  sufficiently  to  break 
the  circuit  and  the  current  falls  to  the  zero  value  at  C.  The 
time  from  C  to  D  is  consumed  in  the  return  of  the  vibra- 
tor under  the  tension  of  the  spring  D  of  Fig.  31  to  that  po- 
sition which  will  close  the  contacts  B  and  C.  Vibrators 


are  usually  made  with  the  contact  B  mounted  on  a  spring 
on  the  armature,  so  that  the  vibrator  is  actually  moving 
toward  the  core  before  contact  B  has  been  pulled  clear  of 
C.  This  permits  of  a  long  "make,"  full  magnetization  of 
the  core,  and  a  "quick  break,"  quick  collapsing  of  the 
field  with  generation  of  maximum  potential  in  the  secon- 
dary. Besides  the  vibrator,  types  of  motor-driven  make 
and  break  devices  may  be  employed  in  the  primary  circuit. 
In  no  case,  however,  is  the  induction  coil  as  satisfactory  as 
the  transformer  which  has  no  moving  parts  to  give  trouble. 

141.  In  the  pioneer  days  of  radiotelegraphy,  alternators 
were  not  common  and  direct  current  was  commonly  ob- 
tained from  batteries.  Accordingly,  the  induction  or  Ruhm- 
korff  coil  (named  after  its  inventor)  was  used  to  obtain  the 
high  potential  necessary  for  charging  the  condensers.  The 
sources  of  high  potential  in  the  Marconi  1896,  the  Lodge 
1898,  and  the  Marconi  1900  transmitters  were  all  induction 
coils.  The  use  of  the  induction  coil  is  now  obsolete  except 
in  occasional  small  installations  on  seaplanes  and  as  emer- 
gency transmitters  aboard  ships. 

87 


142]       ELEMENTS   OF  RADIOTELEGRAPHY. 


142.  The  condenser  F  across  the  contacts  B  and  C  of  Fig. 
31  is  for  the  extinguishing  of  the  spark  across  them. 

XXI. 
CONDENSERS. 

143.  The  condensers  used  in  a  radio  transmitter  must 
be  designed  to  withstand  high  potentials  for  long  periods 
of  use.     Accordingly,  only  the  best  dielectrics  can  be  used, 
and  we  find  condensers  constructed  with  plate  glass  free 
of  lead  and  other  impurities,  mica  (isinglass),  oil  and  com- 
pressed air.     Descriptions  of  the  various  types  are  given 
below. 

144.  Glass  Plate  Condenser.— This  type  of  condenser  en- 
joyed considerable  popularity  in  this  country  for  years  on 
account  of  its  cheapness  and  simplicity  of  construction.     A 

diagram  of  a  single  plate  is  shown  in 
Fig.  33 .  The  metallic  conductor  noted 
in  paragraph  23  is  usually  lead  or  tin 
foil,  fastened  to  the  glass  by  glue, 
turpentine,  white  of  egg,  shellac  or 
some  other  good  adhesive.  The  size 
of  the  tin  foil  is  always  made  less 
than  that  of  the  dielectric  in  order 
that  sparking  cannot  occur  over  the 
edge  of  the  glass.  A  strip  of  tin  foil, 
A,  is  brought  up  to  permit  of  connec- 
tions being  made.  The  capacity  of  a  plate  using  21  ounce 
glass  with  a  metallic  area  of  225  square  inches  is  about  0.002 
microfarads.  It  is  customary  to  connect  such  plates  in 
parallel  so  as  to  get  the  desired  capacity,  as  set  forth  in  para- 
graph 25.  The  potential  across  each  plate  of  a  condenser 
is  reduced  by  connecting  them  in  series.  Thus,  it  is  cus- 
tomary to  reduce  the  potential  across  the  plates  of  a  con- 

88 


Fig.  33- 


CONDENSERS.  [147 

denser  by  connecting  two  banks  of  plates  in  series,  the 
plates  in  each  bank  being  all  in  parallel.  For  example,  if 
there  are  24  plates  each  of  0.002  mf .  capacity  connected  in 
two  series  banks  of  12  each,  the  capacity  of  each  bank  will 
be  0.024  mf.  and  the  capacity  of  the  total  will  be  0.012  mf. 
The  potential  across  each  bank  (and  across  each  plate- 
since  they  are  in  parallel)  will  be  one  half  that  across  the 
total  condenser,  thus  reducing  the  danger  of  breakdown. 

145.  The  intensity  of  an  electrical  field  of  high  potential  is 
greatest  where  the  area  of  the  conductor  is  the  most  lim- 
ited, that  is  to  say,  the  field  is  most  intense  at  all  corners 
and  sharp  turns.     This  intense  field  results  in  the  forma- 
tion of  brush  discharge,  a  sibilant  blue  discharge  at  the  mar- 
gin of  the  tin  foil.     This  brush  discharge  represents  a  loss 
in  the  condenser  and  to  reduce  it,  it  is  customary  to  immerse 
the  plates  in  oil.     While  this  reduces  the  amount  of  the 
brush,  it  tends  to  limit  or  concentrate  the   brush  to  the 
exact  margin  of  the  tin  foil  instead  of  allowing  it  to  creep 
over  the  surface  for  a  short  distance.     This  concentration 
of  the  brush  tends  to  cut  a  groove  in  the  glass  around  the 
margin  and  eventually  results  in  puncture.     This  effect  of 
the  brush  discharge  in  oil  is  easily  observed  after  plates  so 
immersed  have  been  in  use  for  some  time. 

146.  Oil  Condenser. — The  use  of  metal  plates  immersed 
in  oil,  utilizing  the  oil  only  for  the  dielectric,  is  possible  but 
has  not  met  with  any  great  commercial  adoption. 

147.  Compressed  Air   Condenser. — As   noted  in  para- 
graph 105,  compressed  air  is  an  excellent  dielectric  and  as 
such  is  used  in  the  construction  of  condensers.     Two  sets 
of  interleaved  metal  plates,  carefully  insulated  from  each 
other,  are  located  in  a  steel  container  or  tank  within  which 
the  air  is  pumped  to  a  pressure  of  from  160  to  250  pounds 

89 


148]       ELEMENTS   OF  RADIOTELEGRAP HY. 

per  square  inch.  While  air  at  such  pressure  has  a  high 
breakdown  or  disruptive  strength,  its  specific  inductive 
capacity  is  not  much  greater  than  its  value  at  atmospheric 
pressure,  so  that  for  the  average  capacity  used  in  a  radio 
transmitter,  these  condensers  are  quite  large  and  cumber- 
some. They  have  the  advantage,  however,  as  does  the  oil 
condenser,  of  being  self-healing — that  is  to  say,  if  the 
dielectric  punctures,  new  air  rushes  in  to  fill  and  heal  the 
punctured  area.  If  the  dielectric  in  a  glass  condenser 
punctures,  the  condenser  is  rendered  inoperative  until  the 
damaged  plate  has  been  removed. 

148.  Condensers  are  often  fitted  with  safety  gaps,  i.e.,  a 
spark  gap  connected  across  the  terminals  and  set  at  a  cer- 
tain distance  such  that  potentials  which  might  puncture 
the  condenser  will  cause  a  spark  to  jump  the  gap,  short 
circuiting  the  condenser  and  protecting  it. 

149.  Navy  Condensers. — The  type  of  condenser  which 
is  standard  in  the  Navy  is  a  Leyden  jar  (see  paragraph  23 
and  Fig.  3)  whose  walls  are  perpendicular  to  its  base  and 
the  coatings  of  which  are  copper  electrolytically  deposited  on 
the  glass.     The  process  of  manufacture  is  as  follows:    a 
coating  of   graphite  or  aluminum   paint  is  placed  on  the 
inside  and  outside  of  the  jar  as  far  up  as  it  is  desired  to 
plate  it      With  this  graphite  coating  as  one  electrode,  the 
jar  is  immersed  in  an  electro-plating  bath  and  the  copper 
deposited  thereon  by  electrical  means.     Such  jars  have 
capacities  of  either  0.002  or  0.003  mf .     This  type  of  coat- 
ing has  the  advantage  of  not  blistering,  in  marked  contrast 
to  tin  foil,  which,  after  long  use,  blisters  away  from  the 
glass.     Undue  electrical  strain  then  takes  place,  eventually 
leading  to  heating  and  puncturing  of  the  glass. 

150.  In  order  to  limit  the  brush  discharge  area  to  as 
small  a  value  as  possible,  it  is  customary  to  build  jars  of  very 

90 


CONDENSERS. 


[152 


much  greater  length  than  diameter,  and  the  brush  discharge 
area  is  restricted  to  a  little  more  than  12  inches,  as  against 
the  60  inches  or  more  of  the  average  plate  condenser.  The 
Telefunken  Company  has  made  extensive  use  of  this  prin- 
ciple, and  the  Marconi  Company  likewise  in  its  Shoemaker 
jars. 

151.  An  interesting  type  of  condenser  for  the  elimina- 
tion of  the  strains  at  the  margin  of  the  plates  is  the  Moscicki 
condenser  shown  in  Fig.  34.     The  thickness  of  the  glass 
is  increased  at  the  margin  of  the  metallic 

coating  where  the  greatest  strain  occurs, 
and  brush  losses  and  danger  of  puncture 
are  greatly  reduced.  This  type  of  con- 
denser was  used  by  the  De  Forest  Radio 
Company  in  its  quenched  gap  installa- 
tions of  1909  and  1910,  and  by  several 
radio  companies  in  France  and  Germany, 
and  proved  very  successful. 

152.  Mica  Condensers. — This  type  of 

condenser  is  coming  into  increased  pop- 
Fig.  34.  Moscicki 
ulanty,  but  on  account  of  the  high  cost          Condenser 

of  mica  will  doubtless  enjoy  but  a  lim- 
ited adoption.  It  is  similar  to  the  glass  plate  condenser, 
except  that  the  tin  foil  or  copper  plates  are  not  fastened  by 
an  adhesive  to  the  dielectric,  hi  this  case  sheet  mica.  A 
common  size  is  one  with  sheets  six  inches  square,  the  metal 
sheets  being  considerably  smaller  to  avoid  sparking,  as 
noted  in  paragraph  144.  The  alternate  mica  and  metal 
sheets  are  piled  together  and  clamped,  making  a  compact 
condenser  of  high  capacity.  This  type  is  ordinarily  used 
in  low  potential  transmitters,  such  as  the  modern  impulse 
types,  and  has  an  additional  advantage  of  being  less  sub- 
ject to  internal  losses  than  the  glass  plate  type. 

91 


153]       ELEMENTS   OF  RADIOTELEGRAPHY. 

XXII. 
MODERN   SPARK   GAPS. 

153.  The  spark  gaps  of  modern  transmitters  are  divided 
into  the  following  classes:    open,  quenched,  impulse,  and 
rotary.     They  are  discussed  below. 

154.  Open.— This  type  of  gap  is  similar  to  that  shown  in 
Fig.  20,    consisting  of  two  electrodes  or  sparking  surfaces 
facing  each  other  and  adjusted  so  as  to  permit  of  sparking 
between  them.     As  discussed  in  Chapter  Four,  this  type 
of  gap  is  not  easily  deionized,  is  noisy  in  operation,  and 
accordingly  is  becoming  practically  obsolete  in  modern 
installations.     It  is  the  type  of  gap  shown  in  the  early  Mar- 
coni patents. 

155.  Rotary. — The  rotary  spark  gap  may  be  divided  into 
two  classes:  the  synchronous  and  the  non-synchronous. 
It  consists  (as  shown  in  Fig.  35),  of  an  insulating  disk,  D, 
mounted  on  the  shaft  of  some  revolving  machine,  around 
which  disk  is  shrunk  a  metal  hoop   C.     Fastened  equi- 
distantly  around  the  periphery  or  circumference  of  the 
disk  are  a  number  of  electrodes  or  studs.     When  the  po- 
sition of  the  wheel  is  such  that  a  pair  of  studs  comes  on  a 
line  with  the  electrodes  A  and  B,  a  spark  jumps  between 

these  electrodes  and  the  studs 
nearest  them.  There  are  thus 
two  sparks  in  series,  the  cur- 
rent passing  from  one  stud  to 
the  other  by  means  of  the  metal 
hoop  on  the  periphery.  The  ro- 

Fig.  35.  Rotary  Spark  Gap.  tary  gap  has  to  some  extent  the 

characteristics  of  the  quenched 

gap  in  having  two  sparks  in  series  (see  paragraph   103). 

In  addition,  the  rotation  of  the  gap  enhances  the  diffusion 

92 


MODERN  SPARK   GAPS.  [157 

of  the  ions  by  the  windage  (see  paragraph  102),  and  serves 
to  cool  it  (see  paragraph  101).  However,  the  impression 
should  not  be  gathered  that  the  rotary  gap  is  comparable 
with  the  quenched  gap  in  damping  the  current  oscillations 
in  the  gap  circuit,  altho  it  is  considerably  more  efficient 
in  this  regard  than  the  open  type  of  gap. 

156.  In  the  synchronous  gap,  the  stationary  electrodes 
are  so  mounted  as  to  permit  of  sparking  only  when  the  cur- 
rent in  the  secondary  circuit  is  at  the  maximum  values  of 
the  cycle,  at  which  time  the  condenser  is  fully  charged. 
To  effect  this,  the  rotating  disk  is  mounted  on  the  shaft  of 
the  alternator  producing  the  current  which  charges  the 
condenser.     The  gap  is  thus  always  in  time,  or  step,  or 
synchronism,  with  the  supply  current.     To  facilitate  the 
adjustment  of  the  gap  so  as  to  permit  the  condenser  to 
discharge  only  at  the  peak  of  the  cycle  without  precalcu- 
lation, the  stationary  electrodes  A  and  B  of  Fig.  35  are 
mounted  on  a  rocker  arm  which  is  adjusted  until  the  spark 
is  the  clearest  and  smoothest.     Besides  the  increased  ef- 
ficiency of  permitting  the  condenser  to  discharge  only  at 
the  proper  time,  this  type  of  gap  gives  a  pleasing,  musical 
tone  to  the  spark  note,  which  condition  enhances  favorable 
reception  at  the  receiving  station. 

157.  With  the  non-synchronous  gap,  on  the  other  hand, 
the  disk  is  not  driven  in  step  with  the  alternating  current, 
so  that  the  condenser  discharges  at  various  points  on  the 
secondary  current  cycle.     This  results  in  a  slightly  dis- 
cordant note,  composed  of  fundamental  tones  coupled  with 
a  number  of  overtones,  altho  the  high  speed  of  the  disk  and 
the  regular  spacing  of  the  studs  give  a  more  pleasing  tone 
than  with  the  open  gap.     The  efficiency  of  this  type  of  gap 
is  of  course  not  equal  to  that  of  the  synchronous  type,  for 
no  attempt  is  made  to  cause  the  condenser  to  discharge 

93 


158]       ELEMENTS   OF  RADIOTELEGRAPHY. 

at  any  particular  instant.  The  disk  for  this  gap  is  usually 
mounted  on  a  separate  motor.  The  greater  the  speed  of 
the  motor,  the  higher  will  be  the  note  or  pitch  of  the  spark. 

158.  Quenched. — This  type  of  gap  has  been  discussed 
in  Chapter  Four. 

159.  Impulse. — This    type    of    gap    is    similar    to    the 
quenched  gap  except  that  since  the  quenching  in  the  gap 
circuit  of  an  impulse  transmitter  has  to  be  even  more 
perfect  than  that  in  a  quenched  transmitter  in  order  to 
damp  the  oscillations  to  a  single  half  swing  of  current,  it 
must  be  designed  to  quench  very  effectively.     The  author 
has    designed    a   gap,    incorporating   large    copper    disks 
revolving  with  extremely  minute  separation  in  an  atmos- 
phere of  hydrogen  vapor  under  pressure,  which  has  met 
with  some  adoption  in  the  commercial  field.     (See  para- 
graph 105.) 

160.  Rotary  Quenched.— As  may  be  imagined  from  its 
name,  this  type  of  gap  is  a  combination  of  both  the  quenched 
and  rotary  types,  incorporating  the  cooling  effect  of  the  re- 
volving gap  by  not  permitting  the  spark  to  stay  in  any  one 
place  on  the  surface  of  the  disks  but  causing  it  to  wander 
over  the  entire  surface,  with  the  many  beneficial  features 
of  the  quenched  gap.     In  the  gap  described  in  the  pre- 
ceding  paragraph,  the  quenched   gap   disk   is  caused  to 
revolve  at  a  speed  of  1,800  to  3,600  revolutions  per  minute. 
With  the  rotary  quenched  gap  of  the  Japanese  Department 
of  Communications,  the  disks  revolve  very  slowly,  about 
four  revolutions  per  minute. 

XXIII. 
TRANSMITTING  INDUCTANCES. 

161.  In  order  that  oscillations  may  be  produced  in  the 
antenna  circuit  by  the  condenser  discharge  in  the  gap  cir- 

94 


If 


PLATE   IV.     Stone  Impulse  Rotary  Spark  Gap. 
(See  Pars.  105,  159,  160,  205.) 


TRANSMITTING   INDUCTANCES.  [162 

cuit,  whether  this  discharge  be  oscillatory  or  of  the  impulse 
type,  some  form  of  electro-magnetic  coupling  is  usually  em- 
ployed. This  coupling  may  be  inductive  or  conductive  as 
defined  in  paragraph  75.  The  gap  and  antenna  circuit 
windings  of  the  antenna  coupler  are  wound  either  in  cylin- 
drical or  spiral  fashion,  as  shown  in  Fig.  36.  If  inductive 


Fig.  36. 

coupling  is  desired  between  two  cylindrical  coils,  they  are 
placed  end  to  end,  or  one  is  made  of  smaller  diameter  than 
the  other  so  as  to  permit  their  being  telescoped.  Coupling 
between  two  spiral  coils  is  obtained  by  placing  their  faces 
parallel  to  each  other.  The  coupling  between  both  sets  of 
coils  is  weakened  by  drawing  them  apart,  or  by  turning 
one  of  them  thru  an  angle  of  90°.  If  the  two  coils  are  at 
right  angles,  there  will  be  no  inductive  coupling  between 
them  since  the  lines  of  force  set  up  by  the  one  will  not 
thread  the  other  in  a  direction  parallel  to  its  axis.  Even  at 
right  angles  there  is  still  slight  coupling  between  the  two 
coils,  but  this  is  static  coupling — by  virtue  of  their  prox- 
imity. (See  paragraph  75.) 

162.  Fig.  37  shows  how  the  coupling  between  the  two 
circuits  can  be  varied  with  conductive  coupling,  (a)  shows 
medium  coupling  with  two  turns  in  common  between  the 
two  circuits  while  (b)  represents  loose  coupling  since  there 
are  no  turns  in  common  between  the  circuits.  This  type 

95 


163]       ELEMENTS   OF  RADIOTELEGRAPHY. 

of  antenna  coupler  is  quite  commonly  used  in  the  quenched 
gap  transmitters  of  the  Telefunken  Company,  where  close 
coupling  is  desired  in  order  that  the  high  damping  of  the 


Gap  Circuit  ^^\ 


.  Gap  Circuit 
J.  Antenna  Circuit 
(a) 


(b)  *~J    An  tenna  Circuit 

Fig.  37- 

gap  circuit  may  be  assisted  by  a  rapid  transfer  of  energy 
from  that  circuit  to  the  antenna.  The  more  quickly  the 
gap  circuit  gives  up  its  energy  to  the  antenna  circuit,  the 
sooner  will  the  current  swings  in  the  former  circuit  come 
to  rest. 

163.  Antenna  couplers  are  made  of  bare  wire  or  strip  in 
order  that  free  access  may  be  had  to  all  parts  of  the  coils 
so  that  the  wave  length  in  each  circuit  may  be  changed  for 
tuning  purposes.     In  some  types  of  impulse  transmitters, 
operated  on  the  principle  of  the  Lodge  1898  transmitter, 
there  is  no  necessity  for  changing  the  time  period  of  the 
impulse  or  gap  circuit,  and  the  winding  for  that  circuit  may 
thus  be  insulated.     When  bare  or  uninsulated  conductor 
is  used,  the  wire  is  wound  on  porcelain  or  some  other 
insulating  material,   such   as   hard   rubber,   micarta  and 
electrose. 

164.  High  or  radio  frequency  currents  exhibit  the  peculiar 
faculty  of  traveling  near  the  surface  of  the  conductor  and  do 
not  penetrate  into  the  center  of  the  wire  as  with  audio 
frequency  and  direct  currents.     Accordingly,  hollow  metal 
tubes  of  adequate  surface  have  no  more  resistance  to  high- 
frequency  currents  than  a  solid  conductor  of  equal  surface. 
If  the  conductor  be  stranded  and  each  wire  insulated  from 

96 


PLATE  V. 

Antenna  Loading  Inductance,  5-kw.  Transmitter. 
Federal  Telegraph  Co. 


PLATE  VI. 

Antenna  Loading  Inductance,  30-kw.  Transmitter. 
Federal  Telegraph  Company. 


TRANSMITTING  INDUCTANCES.  [166 

the  adjoining  wire  by  a  silk  or  enamel  covering,  the  radio 
frequency  current  will  flow  thru  the  wires  in  the  center  of  the 
cable  as  well  as  those  on  the  outside.  Thus  for  a  stranded 
cable  of  given  diameter,  the  resistance  will  be  very  much 
less  than  that  of  a  solid  conductor  of  equal  diameter,  for 
with  the  stranded  wire,  the  currents  are  traveling  on  the  sur- 
face of  each  individual  wire  since  they  are  all  insulated 
from  each  other.  Such  insulated  stranded  wire  is  termed 
ulitzendraht,"  and  on  account  of  its  exceedingly  low  radio- 
frequency  resistance  is  commonly  used  in  both  transmitter 
and  receiver  circuits.  The  radio-frequency  resistance  of 
all  but  very  small  wires  is  thus  seen  to  be  considerably 
higher  than  its  ohmic  or  low-frequency  resistance.  To  re- 
duce their  radio-frequency  resistances,  transmitter  induct- 
ances are  wound  with  thin  copper  strip  of  large  surface, 
with  litzendraht,  or  with  large  copper  tubing. 

165.  Besides  the  inductances  forming  the  gap  and  an- 
tenna circuit  windings  of  the  antenna  coupler,  additional 
inductances  are  also  inserted  in  the  antenna  circuit  to  in- 
crease its  wave  length.     These  inductances  may  be  wound 
in  cylindrical  or  spiral  fashion  and  the  same  precautions 
as  to  the  reduction  of  resistance  and  the  enhancing  of  insu- 
lation are  observed  as  in  the  types  just  described.     Such 
coils  are  supplied  with  means  for  connecting  more  or  less 
turns  of  their  inductance  in  the  circuit  so  as  to  increase  or 
decrease  the  wave  length.    They  are  termed  loading  coils. 

166.  The  Telefunken  Company  uses  an  unique  type  of 
antenna  inductance,  called  the   variometer,  consisting  of 
two  spiral  inductances  which  are  wound  in  opposite  direc- 
tions and  connected  in  series.     When  these  two  coils  are 
placed  face  to  face,  the  effect  of  the  difference  in  the  direc- 
tion of  winding  is  such  as  to  render  their  total  inductance 
nil,  for  the  lines  of  force  set  up  by  the  one  are  counter  to 

8  97 


167]       ELEMENTS   OF  RADIOTELEGRAPHY. 

those  set  up  by  the  other  and  accordingly  cancel.  The 
two  coils  forming  the  variometer  are  hinged  and  as  one  is 
swung  away  from  the  other,  describing  a  right  angle,  the 
nullifying  effect  of  their  opposite  windings  vanishes  so  that 
when  they  are  at  right  angles,  their  resultant  inductance  is 
twice  that  of  each  taken  separately,  since  in  this  position 
there  is  no  counter  or  "bucking"  effect  of  their  respective 
magnetic  fields.  By  a  slight  movement  of  one  of  the  coils, 
therefore,  a  very  fine  variation  of  the  inductance,  and  hence 
the  wave  length,  of  the  antenna  circuit  may  be  obtained. 
The  variometer  has  the  disadvantage,  however,  of  having 
much  higher  resistance  per  unit  of  inductance  than  the 
ordinary  loading  coil.  In  the  case  when  the  coils  of  the 
variometer  are  most  closely  coupled,  for  instance,  for  the 
fairly  large  resistance  of  the  two  coils  in  series  there  is  a 
zero  value  of  inductance. 


XXIV. 
ANTENNA   CURRENT   AMMETER. 

167.  In  any  one  particular  transmitter,  the  amount  of 
current  flowing  in  the  antenna  circuit  is  usually  indicative 

of  the  performance  of  the 
apparatus.  To  measure 
this  current,  an  ammeter 
is  inserted  in  the  antenna 
circuit  in  the  position  shown 
in  Fig.  38,  where  A  repre- 
sents the  antenna  amme- 
ter. The  type  of  ammeter 
used  in  the  measurement 
of  audio  frequency  or  di- 
rect currents  is  not  suitable 
Fig.  38.  for  the  measurement  of 

98 


ANTENNA  CURRENT  AMMETER.          [167 

current  of  radio  frequency  so  a  special  type  of  meter  is  em- 
ployed. It  is  constructed  as  follows :  A  fine  wire  is  stretched 
between  the  two  supports  A  and  B  of  Fig.  39,  thru  which  a 
certain  portion  of  the  antenna  current  to  be  measured  is 
passed.  This  current  in  overcoming  the  resistance  of  the 


Fig.  39.     Hot- Wire  Ammeter. 

wire  heats  it,  just  as  the  filament  of  an  electric  lamp  is 
heated.  A  heated  metal  tends  to  expand,  so  that  the  wire 
AB  assumes  the  position  shown  in  the  dotted  line.  To 
this  wire  AB  at  the  point  E  is  soldered  a  wire  EF.  To 
EF  at  the  position  G  is  fastened  a  fine  silk  thread  which  is 
passed  once  around  the  spindle  C  on  the  indicating  nee- 
dle of  the  instrument.  The  slack  in  this  thread  is  taken 
up  by  the  spring  D.  When  the  wire  AB  is  heated  and 
sags  to  the  position  shown,  the  wire  EF  under  the  strain 
exerted  on  it  by  the  spring  D  thru  the  thread  assumes 
the  position  shown  in  the  dotted  lines.  Spring  D,  hi 
taking  up  the  sag  of  the  system  caused  by  the  expansion 
of  AB  from  the  heat  generated  by  the  antenna  current, 

99 


168]       ELEMENTS   OF  RADIOTELEGRAPHY. 

causes  the  thread  around  the  spindle  C  to  revolve  it,  mov- 
ing the  needle  to  the  position  shown  in  the  dotted  line.  The 
amount  of  heat  generated  is  proportional  to  the  square  of 
the  antenna  current  (see  equation  4),  so  that  the  greater 
the  current  passed  thru  the  instrument,  the  greater  will  be 
the  heat  generated,  the  greater  the  expansion  of  the  wire  AB 
and  consequently  the  greater  the  deflection  of  the  needle. 

168.  The  wire  used  in  this  hot-wire  ammeter  is  too  small 
to  take  the  entire  antenna  current,  so  it  is  customary  to 
shunt  or  bridge  it  with  heavy  wire  to  carry  the  major  por- 
tion of  the  current.     (In  actual  practice,  it  is  rather  better 
to  make  the  shunt  of  several  wires  in  parallel,  each  wire  of 
the  same  size  as  the  heating  element  AB.    This  procedure 
insures  the  passing  of  a  constant  proportion  of  current  thru 
the  shunt  for  any  frequency,  for  the  skin  effect  of  the  high 
frequency  current — discussed  in  paragraph   164 — will  be 
the  same  on  all  of  the  wires  at  any  particular  frequency. 
If  this  is  not  done  and  a  solid  conductor  is  used,  the  am- 
meter will  only  be  accurate  for  the  particular  frequency  or 
wave  length  at  which  it  was  calibrated  or  graduated.)     The 
use  of  the  antenna  ammeter  will  be  discussed  in  Chapter 
Seven  on  wave  meters  and  tuning. 

169.  Another  type  of  radio  frequency  ammeter  makes 
use  of  the  potential  generated  at  the  heated  juncture  of 


Fig.  40. 

two  dissimilar  metals.  A  portion  of  the  antenna  current 
is  passed  thru  the  wires  BA  and  AC  which  are  joined  to- 
gether at  A  of  Fig.  40.  These  wires  are  made  of  different 

100 


PLATE  VII.     Antenna-current  Ammeter  and  Heating  Element. 
Weston  Electrical  Instrument  Co. 


ANTENNA    CONDENSER:  [170 


substances,  such  as  antimony  and  bismuth,  or  iron  and 
constantan  (a  nickel  alloy).  The  antenna  current  heats 
this  juncture,  causing  a  slight  potential  to  be  generated 
which  is  measured  by  the  voltmeter  V.  (The  voltmeter 
has  too  high  a  resistance  and  reactance  to  the  radio  fre- 
quency current  to  be  affected  by  it.)  The  voltmeter  is 
calibrated  in  amperes,  indicating  the  amount  of  current  in 
the  antenna  circuit  which  produced  the  heating  of  the 
thermo-couple.  Thus,  while  this  instrument  is  called  an 
antenna  current  ammeter,  it  is  hi  reality  a  voltmeter,  meas- 
uring the  potential  generated  at  the  thermo-couple  by  the 
antenna  current.  This  type  of  radio  frequency  ammeter 
is  quite  accurate  and  is  coming  into  wide  adoption. 

XXV. 
ANTENNA   CONDENSER. 

170.  The  wave  length  of  the  antenna  circuit  without 
loading  inductance  therein,  that  is  to  say,  the  antenna  con- 
nected to  the  earth  with  a  wire,  is  termed  its  fundamental 
or  natural  wave  length.  In  order  that  oscillations  may  be 
produced  in  the  antenna  circuit,  it  is  necessary,  as  we  have 
previously  observed,  to  insert  sufficient  inductance  in  the 
circuit  to  form  the  antenna  circuit  winding  of  the  coupler, 
by  means  of  whose  mutual  inductance  energy  from  the 
gap  circuit  is  received  therein.  This  inductance  adds  from 
25  to  50  meters  to  the  natural  wave  length  of  the  antenna 
circuit,  and  cannot  be  dispensed  with.  Let  us  suppose 
that  we  have  an  antenna  whose  natural  wave  length  is 
350  meters  and  the  antenna  coupler  inductance  adds  an- 
other 30  meters,  making  a  total  of  380  meters.  Let  it  be 
desired  to  radiate  a  wave  length  of  325  meters.  What  will 
be  the  procedure? 

101 


171]       ELEMENTS   OF  RADIOTELEGRAPHY. 

171.  We  have  observed  in  paragraphs  21  and  25  that  if 
two  condensers  are  connected  in  series,  the  resultant  cap- 
acity will  be  less  than  the  capacity  of  either  of  them.  Sim- 
ilarly, the  capacity  of  an  antenna  (see  paragraph  70)  may 
be  reduced  by  connecting  a  condenser  in  series  with  it  as 


Fig.  41.     Connection  of  Antenna  Condenser. 

in  Fig.  41.  Since  the  capacity  is  reduced,  the  wave  length 
as  well  is  decreased,  for  the  wave  length  is  proportional  to 
the  product  of  the  inductance  and  capacity,  as  previously 
discussed.  The  use  of  a  series  condenser  thus  permits  of 
the  adjustment  of  the  antenna  circuit  to  a  wave  length 
shorter  than  its  fundamental. 

172.  For  this  type  of  condenser,  it  is  customary  to  use 
two  plates  or  jars  in  series — in  order  to  reduce  the  danger 
of  puncture,  for  if  the  condenser  should  break  down,  no 
warning  would  be  given,  the  condenser  would  be  short 
circuited  by  the  ensuing  spark  occurring  within  the  glass, 
and  the  antenna  circuit  would  have  the  wave  length  to 
which  it  was  tuned  prior  to  the  insertion  of  the  condenser. 
This  would  throw  the  gap  and  antenna  circuits  out  of  re- 

102 


ANTENNA   SWITCH. 


[174 


sonance,  with  the  consequent  emission  of  an  impure  wave. 
(See  paragraph  110). 

XXVI. 


ANTENNA   SWITCH. 


173.  In  small  radio  installations,  that  is  to  say,  in  those 
aboard  ship  and  at  small  coastal  stations,  the  same  an- 
tenna is  used  for  transmitting  as  for  receiving,  In  order 
to  throw  the  antenna  from  the  one  circuit  to  the  other,  a 


Fig.  42.     Connection  of  Antenna  Switch. 

switch  termed  the  antenna  switch  is  provided.  It  is  shown 
in  its  simplest  form  in  Fig.  42,  being  merely  a  single-pole, 
triple-throw  switch,  arranged  to  connect  the  antenna  alter- 
nately to  the  transmitter  or  receiver  as  desired.  This 
switch  is  also  arranged  to  connect  the  antenna  to  earth  in 
the  event  of  violent  electrical  storms— thus  serving  to  safe- 
guard the  operator  and  the  apparatus. 

174.  More    elaborate    antenna    switches    are    provided 
which  combine  with  the  opening  and  closing  of  the  antenna 

103 


175]       ELEMENTS  OF  RADIOTELEGRAPHY. 

circuit  in  either  the  transmitter  or  receiver,  the  control  of 
additional  circuits  such  as  the  blower  or  rotary-spark-gap 
motor  circuits  in  the  transmitter  and  the  telephone  and 
detector  circuits  in  the  receiver. 

175.  When  attachments  are  fitted  to  the  transmitter  key 
to  provide  for  the  opening  and  closing  of  the  antenna  circuit 
coincident  with  the  operation  of  the  key,  it  is  called  a  break 


Fig.  43.     Break-Key. 

key.  A  diagram  is  shown  in  Fig.  43.  The  contacts  A  and 
B  are  the  ordinary  key  contacts  for  making  and  breaking 
the  current  into  the  primary  of  the  step-up  transformer. 
The  contacts  C  and  D  are  connected  across  the  terminals 
of  the  receiving  apparatus  so  as  to  short  circuit  it  and  pre- 
vent it  from  injury  from  the  heavy  antenna  current  of  the 

104 


PLATE  VIII.    Electrically-operated  Antenna  Switch  and  Wave  Changer, 

35o-kw.  Transmitter. 
Federal  Telegraph  Company. 


ANTENNA   SWITCH.  [177 

transmitter.  In  the  depression  of  the  key,  contacts  C  and 
D  are  adjusted  so  as  to  close  before  A  and  B.  This  in- 
sures complete  protection  to  the  receiver  before  the  current 
is  allowed  to  flow  in  the  transmitter.  When  the  key  is 
not  depressed  as  shown  in  the  figure,  receiving  is  done 
with  the  antenna  circuit  winding  of  the  transmitter  antenna 
coupler  in  series  with  the  primary  of  the  receiver.  When 
the  key  is  depressed,  the  receiver  is  short  circuited  by  the 
contacts  C  and  D  and  rendered  inoperative,  and  the  trans- 
mitter is  placed  in  operation.  In  the  process  of  signalling, 
the  key  is  up  more  often  than  it  is  depressed,  permitting 
the  operator  to  receive  in  fragmentary  fashion  while  he  is 
sending.  The  break-key  has  the  great  advantage  of  per- 
mitting the  receiving  operator  to  interrupt  or  "break-in" 
on  the  transmitting  operator,  for  if  the  former  fails  to 
receive  a  character  he  has  merely  to  close  his  key,  sending 
out  a  long  dash,  which  the  transmitting  operator  will  hear 
upon  the  next  instant  that  his  key  is  in  the  "up"  position. 

176.  The  break-key  is  thus  an  automatic  antenna  switch 
and  is  a  very  efficient  piece  of  apparatus,  especially  in 
fleet  operation,  for  if  all  the  vessels  of  the  fleet  are  equipped 
with  it,  it  is  possible  for  the  flagship  to  "pipe  down"  all  ships 
at  any  time  in  order  to  send  a  general  order. 

177.  The  use  of  separate  sending  and  receiving  antennae 
at  high  power  stations  results  in  the  same  advantage  of 
being  able  to  transmit  and  receive  simultaneously,  altho 
the  latter  procedure  is  even  more  perfect — and  more  ex- 
pensive—than the  break-key. 


105 


CHAPTER   SIX. 

XXVII. 
COMPLETE   TRANSMITTER. 

178.  A  diagram  of  a  complete  transmitter  from  the  service 
mains  thru  to  the  aerial  and  ground  is  given  in  Fig.  44. 


Fig.  44.     Complete  Circuit  of  a  Transmitter. 

179.  A  compound- wound  motor,  having  series  and  shunt 
fields  is  shown.  When  a  motor  is  running,  it  generates  a 
back  or  counter  electromotive  force,  opposite  in  direction 
to  the  potential  which  causes  it  to  revolve.  The  resultant 
potential  across  the  terminals  of  the  machine  is  the  differ- 
ence between  the  applied  E.M.F.  and  the  counter  E.M.F. 
When  the  motor  is  stationary,  no  such  counter  E.M.F.  is 

106 


COMPLETE   TRANSMITTER.  [182 

generated,  and  if  resistance  is  not  inserted  in  series 
with  the  armature  or  rotor,  damage  to  the  winding  will 
occur  in  starting  it.  The  starting  resistance  B  is  provided 
with  several  buttons  over  which  a  contacting  handle  is 
moved.  No  such  protection  is  necessary  for  the  motor 
field  since  it  has  a  resistance  sufficiently  high  to  prevent 
a  dangerously  large  current  from  flowing  thru  it. 

180.  A  hold-over  magnet  is  provided  on  the  starting  box. 
On  the  handle  is  fastened  a  small  yoke  of  iron.     When  the 
handle  reaches  the  last  button,  the  magnet  A  attracts  this 
iron  yoke,  and  holds  the  handle  against  the  tension  of  a  spring 
tending  to  pull  it  back.    Such  a  device  is  termed  a  no-voltage 
release,  for  should  the  potential  be  cut  off  the  line  for  any 
reason,  the  magnet  A,  losing  its  magnetism,  releases  the 
handle,  making  it  necessary  to  go  thru  the  operation  of 
starting  the  motor  when  the  potential  comes  back  on  the 
line.     If  this  device  were  not  provided  and  the  potential 
should  go  off,  the  motor  would  stop  running,  and  when  the 
voltage  were  cut  in  again  there  would  be  no  resistance  in 
series  with  the  armature  to  protect  it. 

181.  The  compound- wound  motor  has  the  advantage  of 
maintaining  a  constant  speed  and  hence  assures  practically 
constant  potential  and  frequency  on  the  part  of  the  alter- 
nator under  the  fluctuating  load  of  signalling. 

182.  We  have  observed  in  equations  (13)  and  (14)  that 
the  potential  and  frequency  of  an  alternator  are  propor- 
tional to  its  speed.     Raising  the  speed  of  the  motor,  to 
which  the  alternator  is  directly  connected,  will  result  in  an 
increase  of  frequency  and  potential.     The  potential  of  the 
alternator  also  depends  upon  the  strength  of  the  magnetic 
flux  of  the  poles  (see  paragraphs  30  and  32),  which  is  in- 
creased by  increasing  the  amount  of  current  flowing  thru 
the  field  CPlls.     This  field  is  excited  by  current  from  the 

107 


183]       ELEMENTS   OF  RADIOTELEGRAPHY. 

service  mains  thru  the  variable  resistance  RG.  As  more 
and  more  resistance  is  cut  out  of  this  rheostat,  more  cur- 
rent flows  thru  the  alternator  field  with  a  consequent 
increase  in  its  generated  potential. 

183.  Since  the  greater  the  current  thru  a  generator  field, 
the  greater  the  potential  which  it  produces  at  a  certain 
speed,  it  follows  that  the  greater  the  current  thru  the  field 
of  a  motor,  the  greater  will  be  its  counter  E.M.F. — the 
potential  which  it  produces.    The  greater  the  counter  E.M.F. 
of  a  motor,  the  less  will  be  the  current  flowing  thru  the 
motor  armature.    Thus,  to  reduce  the  speed  of  a  shunt- 
wound  motor,  less  resistance  should  be  inserted  in  series 
with  the  field  coils  so  as  to  increase  the  strength  of  the 
magnetic  field,  for  then  the  speed  need  not  be  so  high  to 
generate  the  proper  amount  of  counter  E.M.F.;  and  con- 
versely, to  make  it  run  faster,  an  increase  in  the  field  re- 
sistance will  allow  less  current  to  flow  thru  the  field  and  the 
motor  must  revolve  faster  to  generate  the  counter  E.M.F. 

184.  As  noted  in  paragraph  182,  the  alternator,  usually 
500  cycles,  for  the  generation  of  the  alternating  current  for 
the  radio  transmitter,  is  directly  coupled  to  the  motor,  in 
fact — the  armatures  of  the  two  machines  may  be  mounted 
on  the  same  shaft.     A  voltmeter,  Vy  connected  across  its 
terminals  serves  to  measure  its  potential,  an  ammeter,  in 
series  with  the  primary  of  the  transformer,  indicates  the 
current,  and  a  wattmeter  W  measures  the  power.    The 
wattmeter  reading  divided  by  the  product  of  the  readings 
of  the  voltmeter  and  ammeter  will  give  us  the  power  factor 
as  defined  in  paragraph  42. 

185.  In  every  radio  transmitter,  there  is  great  danger 
from  induction  of  radio  frequency  currents  into  the  low- 
voltage,  low-frequency  circuits.     These  radio  frequency 
currents  are  picked  up  by  the  low-potential  circuits -lying 

108 


COMPLETE    TRANSMITTER.  [187 

near  the  antenna  or  gap  circuits,  and  are  of  such  high  po- 
tential as  to  seriously  endanger  the  primary  of  the  trans- 
former and  the  alternator,  both  of  which  are  not  insulated 
against  more  than  a  comparatively  low  voltage  Besides 
the  induction  between  the  radio  frequency  and  audio  fre- 
quency circuits,  it  is  also  possible  for  radio  frequency  cur- 
rents to  make  their  way  into  the  primary  circuit  by  means 
of  what  is  termed  the  distributed  capacity  of  the  secondary 
of  the  transformer.  (This  term  will  be  discussed  in  a  later 
chapter  on  receivers.)  To  render  such  induced  currents 
harmless,  it  is  customary  to  connect  condensers  of  about 
2.0  microfarads  capacity  each  between  the  line  and  the 
earth.  They  are  shown  in  Fig.  44,  labelled  as  PC.  We 
have  observed  in  paragraph  35  that  the  reactance  of  a  con- 
denser decreases  with  an  increase  in  frequency,  hence 
these  condensers  which  have  a  high  reactance  to  the  pri- 
mary current  of  from  60  to  500  cycles,  and  thus  do  not 
conduct  any  of  the  audio  current  to  the  ground,  have  vir- 
tually no  reactance  to  radio  frequency  currents  of  the  order 
of  500,000  cycles  and  form  practically  a  short-circuit  to 
earth  for  them.  This  grounded  path  serves  to  drain  the 
line  of  these  high  potential  currents  and  obviates  the 
possibility  of  damage  from  them.*  The  use  of  series  in- 
ductances or  choke  coils  for  the  same  purpose  will  be  dis- 
cussed in  Chapter  Eight  on  the  Poulsen  arc. 

186.  The  purpose  of  the  remainder  of  the  apparatus 
shown  in  this  figure  is  obvious  and,  having  been  previously 
discussed,  will  not  be  dealt  with  further. 

187.  There  are  several  systems  of  damped  wave  radio- 
telegraphy  in  use  at  present  and  their  transmitters  will  be 
briefly  described  below. 

*  See  author's  discussion  of  the  subject  in  the  December,  1917> 
issue  of  the  Proceedings  of  the  Institute  of  Radio  Engineers. 

109 


188]       ELEMENTS   OF  RADIOTELEGRAPHY. 

XXVIII. 
MARCONI   SYSTEM. 

188.  As  explained  in  paragraph  110,  the  deleterious  ef- 
fect of  the  tuned  coupled  circuits  of  the  Marconi  1900 
transmitter,   in   the   radiation   of    waves   of    double   fre- 
quency, caused   the   Marconi   Company,   about    1912,   to 
abandon  the  use  of  a  closed  reservoir  gap  circuit  slowly 
feeding  energy  into  the  antenna   circuit,  and   remaining 
coupled   thereto  until   all   of   the   energy   of   the   double 
system  had  been  dissipated  in  the  form  of  heat  and  the 
radiation  of  two  waves,  and  to  substitute  for  the  open  gap 
of  the  Marconi  patent  the  modern  quenched  gap  or  rotary 
(usually  synchronous)  gap.     These  gaps  have  the  faculty, 
as  previously  set  forth,  of  quickly  damping  the  oscillations 
in  the  gap  circuit  and  virtually  uncoupling  it  from  the  an- 
tenna circuit,   leaving  the   antenna   circuit  uto   oscillate, 
free  from  any  disturbance  due  to  maintained   connection 
with  the  source  of  electricity,"  in  the  words  of  the  Lodge 
patent. 

189.  The  circuit  of  the  modern  Marconi  500-cycle  trans- 
mitter is  substantially  that  shown  in  Fig.  44,  except  that 
an  additional  synchronous  rotary  gap  is  provided  to  be 
used  as  a  spare  or  relief  for  the  quenched  gap.     Means  are 
provided  for  quickly  changing  the  wave  length  of  both  the 
antenna  and  gap  circuits,  the  necessity  for  the  critical  tun- 
ing of  which  is  set  forth  in  paragraph  116.     The  sets  are 
mounted  on  panels  permitting  of  quick  and  compact  instal- 
lation, are  quite  efficient,  and  are  admirably  suited  for 
ship  and  small  coastal  station  use.     They  are  more  fully 
described  in  the  August,  1916,  issue  of  the  Proceedings  of 
the  Institute  of  Radio  Engineers. 

110 


TELEFUNKEN  SYSTEM.  [192 

XXIX. 

TELEFUNKEN   SYSTEM. 

190.  The  characteristics  of  this  system  have  substan- 
tially remained  unchanged  since  the  disclosures  of  the 
original  patents  owned  by  the  Telefunken  Company.     It 
embodies  the  use  of  a  500-cycle  quenched  gap  transmitter, 
whose   operating  characteristics  have   been  discussed  in 
Chapter  Four.     This  system  is  exceedingly  efficient  and 
many  sets  were  purchased  before  the  war  from  the  Atlantic 
Communication  Company,  the  American  Telefunken  agents, 
by  the  Army  and   Navy  and  by  commercial  companies. 
This  system  is  used  by  the  German  military  and  naval 
forces  in  shore,  ship,  airplane  and  Zeppelin  installations. 

XXX. 

KILBOURNE    &   CLARK   SYSTEM. 

191.  The  system  of  the  Kilbourne  &  Clark  Manufactur- 
ing Company  of  Seattle  is  of  fairly  recent  design  and  dis- 
tribution, being  first  manufactured  in  1915.     This  company 
uses  two  systems  of  transmitters,  the  Thompson  and  the 
Simpson  Mercury  Valve  (named  after  engineers  of  the 
company.) 

192.  A  diagram  of  the  Thompson  system  is  shown  in 
Fig.  45.     It  is  predicated  or  based  strictly  on  the  Lodge 
patent  of  1898,  which  expired  in  1915  >     It  contains  an  im- 
pulse circuit  C — Q — L — @,  that  is  to  say,  the  condensers 
C,  the  quenched  gaps   Q  and  inductance  L.    In  equation 
(32),  the  necessary  factors  for  high  damping  of  the  current 
in  the  gap  circuit  were  given,  i.e.,  large  capacity,  small  in- 
ductance and  large  resistance.     In  this  system,  a  mica  con- 
denser of  far  more  than  average  capacity,  two  quenched 
gaps  of  high  resistance,  and  an  exceedingly  small  value  of 

111 


193]       ELEMENTS   OF  RADIOTELEGRAPHY. 

gap-circuit  inductance  are  used.  In  order  that  this  induc- 
tance may  be  kept  down  to  the  smallest  possible  limit,  all 
leads  or  connecting  wires  in  the  circuit  are  dispensed  with  by 
mounting  the  quenched  gaps  directly  on  the  condensers 


Fig.  45.     Thompson  Impulse  Transmitter. 

and  utilizing  the  single  turn  of  the  gap  circuit  winding  of 
the  antenna  coupler  to  complete  the  circuit.  The  conditions 
set  forth  in  formula  (34)  for  the  non-oscillatory  discharge 
of  a  condenser  are  thus  realized  and  impulses  instead  of 
oscillations  occur  in  the  gap  circuit.  The  Thompson  trans- 
mitter is  thus  an  impulse  excitation  transmitter — the  first 
American  adaption  of  the  impulse  principles  of  Lodge. 

193.  As  in  the  Lodge  transmitter,  no  necessity  for  res- 
onant tuning  of  the  gap  and  antenna  circuits  arises  for  there 
is  but  a  single  oscillating  circuit — the  antenna  circuit.  The 
gap  circuit  accordingly  has  a  time  period  corresponding  to 
a  wave  length  of  about  700  meters  and  no  provision  exists 
for  changing  it,  none  of  the  parts  of  the  circuit  being  vari- 
able. Sufficient  inductance  is  provided  to  permit  the  an- 
tenna circuit  to  be  tuned  to  a  wave  length  not  exceeding 
600  meters— this  set  is  intended  for  commercial  marine 
installations — and  any  wave  length  down  to  approximately 
250  meters.  Since  this  system  is  a  single  circuit  trans- 

112 


PLATE   X. 

Front  View,  i  kw.  Transmitter  (Simpson  Type). 
Kilbourne  &  Clark  Mfg.  Co. 


PLATE  XI. 

Side    View,    i-kw.    Transmitter    (Simpson  Type). 
Kilbourne    &   Clark  Mfg.  Co. 


KILBOURNE  AND   CLARK  SYSTEM.       [195 

mitter  as  defined  in  Chapter  Three,  it  is  only  necessary  to 
vary  the  adjustments  of  the  antenna  circuit  to  radiate  a 
pure  wave  of  feeble  damping  on  any  wave  length. 

194.  It  is  essential  in  the  operation  of  any  impulse  trans- 
mitter, that  the  frequency  of  the  impulses  shall  not  be  too 
great,  for  if  they  occur  too  rapidly,  there  will  not  be  time 
for  the  antenna  circuit  to  complete  its  feebly  damped  os- 
cillation before  another  impulse  shocks  it  into  vibration 
again.     The  rapidity  with  which  a  condenser  of  given 
capacity   can   be   charged    depends    on    the    voltage    of 
the   secondary  of  the   transformer.     Accordingly,   means 
are  provided  for  accurately  varying  the  potential  in  this 
circuit  by  the  insertion  of  a  variable  resistance  between 
the  secondary  S  of  the  transformer  and  the  condenser  C. 
No  effort  is  made  to  tune  this  condenser  to  resonance 
with  the  charging  circuit,  as  described  in  the  preceding 
chapter. 

195.  A  diagram  of  the  Simpson  Mercury  Valve  Trans- 
mitter is  given  in  Fig.  46.     To  understand  the  operation  of 
this  transmitter,  it  will  be  necessary  to  understand  the 


Fig.  46.     Simpson  Mercury  Valve  Transmitter. 

principle  of  operation  of  the  mercury  valve  rectifier.  This 
instrument  was  invented  by  Cooper-Hewitt.  It  con- 
sists of  a  glass  receptacle  made  in  the  shape  shown  in 

113 


195]       ELEMENTS  OF  RADIOTELEGRAPHY. 

Fig.  47.  A  rectifier  is  an  instrument  which  has  the 
peculiar  property  of  permitting  an  electric  current  to  flow 
thru  it  in  one  direction  only.  Obviously,  such  an  instru- 


Fig.  47.     Mercury  Valve  Rectifier. 

ment  could  not  form  part  of  an  alternating  current  circuit, 
for  its  high  resistance  to  the  passage  of  the  electric  current 
thru  it  in  one  direction  and  its  low  resistance  to  current 
flowing  in  the  opposite  direction  would  not  permit  such  an 

114 


KILBOURNE   AND   CLARK  SYSTEM.        [196 

alternating  current  to  flow.  However,  when  it  is  desired 
to  change  alternating  current  to  direct  current,  the  inser- 
tion of  a  rectifier  in  the  circuit  will  prevent  current  flowing 
except  in  one  direction,  and  while  the  resultant  current  is 
not  of  constant  value,  it  is  uni-directional.  (See  Chap- 


\  /  \  / 

\          /  \          / 

Fig.  48. 

ter  Five  on  the  use  of  the  induction  coil  vibrator.)  Thus, 
one  half  of  each  cycle  of  alternating  current,  represented  in 
the  dotted  line  in  Fig.  48,  is  wiped  out  or  canceled,  and 
only  the  upper  half  can  be  utilized.  Such  resulting  cur- 
rent, as  shown  in  the  heavy  line,  is  termed  pulsating  direct 
current  as  we  have  previously  defined  the  term. 

196.  In  Fig.  47,  the  glass  tube  of  the  rectifier  is  evacu- 
ated or  pumped  to  a  fairly  high  vacuum.  A  small  pool  of 
mercury  is  held  in  the  lower  part  of  the  tube,  at  a  level 
such  that  it  does  not  quite  make  contact  between  the  elec- 
trodes C  and  N.  When  the  tube  is  tilted  to  the  left— this 
act  is  always  necessary  in  starting  a  rectifier  of  this  type — 
the  mercury  completes  the  direct  current  or  "keep-alive" 
circuit,  C,  F  (source  of  B.C.),  RI  and  N.  When  the  tube  is 
again  brought  back  to  the  vertical  position  and  the  mercury 
flows  back  from  the  electrode  C,  instead  of  opening  the 
circuit,  an  arc  or  spark  occurs,  similar  to  the  arc  between 
two  pieces  of  carbon  in  an  arc  light.  The  arc  thus  takes 
place  between  the  pool  of  mercury,  which  is  an  excellent  con- 
ductor of  electricity,  and  the  electrode  C.  A  mercury  arc 
gives  rise  to  a  state  of  high  ionization  within  the  tube,  and 
high-potential  currents  from  the  secondary  of  the  step-up 

115 


197]       ELEMENTS   OF  RADIOTELEGRAPH?. 

transformer  are  enabled  to  flow  from  either  electrode  A  or 
A!  to  the  electrode  N.  The  mercury  arc,  however,  while 
it  will  pass  current  from  A  or  Al  to  N,  will  not  permit  cur- 
rent to  flow  from  N  to  either  of  these  electrodes.  It  is 
thus  seen  to  possess  the  necessary  qualifications  for  a  rec- 
tifier, in  that  its  conductivity  is  unilateral  or  of  but  single 
direction. 

197.  Let  us  assume  that  at  any  instant  the  terminal  B  of 
the  secondary  of  the  transformer  is  positively  charged  and 
that  D  is  negatively  so.     Current  cannot  flow  from  B  thru 
the  highly  ionized  tube,  in  a  direction  from  AI  to  N,  thence 
to  A  and  back  to  D,  for  we  have  just  seen  that  the  arc  will 
not  permit  current  to  flow  from  N  up  the  tube  to  either 
electrode.     Accordingly,  the  current  flows  from  B  to  AI, 
thence  thru  the  arc  to  Nt  thence  thru  the  circuit  represented 
by  #2,  and  back  to  the  transformer  to  the  point  £,  which  is 
in  the  exact  center  of  the  secondary.     At  the  next  half  of 
the  cycle,  the  conditions  are  reversed  and  D  becomes  posi- 
tively charged  and  B  negatively  so.     The  current  then  flows 
from  D  to  A,  thence  to  N  and  thru  the  circuit  /?2>  in  the  same 
direction  as  before,  back  to  E.     E  is  thus  always  negative 
in  respect  to  the  alternate  positive  charges  on  B  and  D,  while 
AT  is  always  positive.     Pulsating  direct  current  flows  in  the 
circuit  RI,  in  the  same  direction  at  all  times.      Neglecting 
the  resistance  of  the  tube,  the  unidirectional  potential  ob- 
tained is  one  half  that  of  the  secondary  of  the  transformer, 
since  only  one  half  of  the  secondary,  either  DE  or  EB  is 
in  use  at  any  one  time. 

198.  In  an  instrument  employing  electrodes,  the  positive 
ones  are  termed  anodes,  while  the  negative  are  called 
cathodes.    In  the  mercury  rectifier,  A  and  AI  are  termed 
the  anodes,  being  the  positive  electrodes  of  the  pulsating 

116 


KILBOURNE  AND   CLARK  SYSTEM.        [200 

direct  current  passing  thru  the  arc,  and  N  is  the  cathode 
since  it  is  connected  thru  R2  to  the  negative  point  E  of  the 
secondary. 

199.  The  reader  may  inquire  as  to  the  necessity  of  so 
complicated  an  apparatus  in  lieu  of  the  use  of  direct  current 
as  obtained  from  a  D.C.  generator.     The  reason  is  because 
that  with  direct  current,  it  is  not  possible  to  raise  or  lower 
the  potential  thru  the  medium  of  the  transformer.     Accord- 
ingly, a  transformer  is  used  to  raise  or  lower  the  alternating 
potential,  as  the  case  may  be — in  this  instance,  the  former- 
after  which  the  rectifier  is  employed  to  change  the  A.C.  to 
D.C.,  if  this  be  desired.     Rectifiers  find  their  most  com- 
mon adoption  in  charging  storage  batteries  when  direct 
current  is  not  available. 

200.  In  the  Simpson  transmitter  of  Fig.  46,  the  "keep- 
alive"  circuit  of  Fig.  47  just  described  is  not  shown.     In 
all  other  respects,  the  circuits  and  operation  of  the  valve 
are  as  given  in  the  preceding  paragraphs.     The  operation 
of  this  transmitter  is  given  by  the  inventor,  F.  G.  Simpson, 
as  follows : 

"Energy  from  the  power  transformer  will  pass  alternately 
thru  the  anodes  A  and  AI  to  the  cathode  N  in  a  uni-direc- 
tional  impulse,  thence  thru  resistance  R  directly  into  the 
antenna.  The  power  supply  circuit  is  completed  by  the 
conductor  from  spiral  W  in  the  antenna  to  the  secondary" 
at  the  point  E. 

"The  antenna  circuit  comprises  the  overhead  wires  A, 
variable  inductance  L,  spiral  inductance  W,  variable  con- 
denser C,  and  the  ground.  The  system  is  so  proportioned 
and  adjusted  that  when  the  antenna  is  fully  charged  the 
mercury  valve  closes,  that  is,  its  resistance  rises  to  a  point 
sufficient  to  prevent  the  formation  of  an  arc  across  the  spark 
gap  <?.» 

117 


200]       ELEMENTS   OF  RADIOTELEGRAPHY. 

"The  energy  is  thus  delivered  to  the  antenna  in  static 
form.  But  radiation  cannot  take  place  until  this  energy 
has  been  set  into  oscillation.  For  this  purpose  what  is  des- 
ignated as  a  converting  trigger  is  utilized.  This  consists 
of  variable  condenser  C,  conductor  2,  special  Simpson 
spark  gaps  (),  inductance  X,  and  a  small  portion  of  the  in- 
ductance W.  The  condenser  C  and  a  small  portion  of 
coil  W  are  common  to  the  antenna  and  the  converting  trig- 
ger. When  the  antenna  is  fully  charged,  the  pressure 
breaks  down  the  resistance  of  the  spark  gaps  Q  and  a  por- 
tion of  the  current  flows  thru  the  spark  gaps  in  the  convert- 
ing trigger  and  is  set  into  oscillation.  The  converting 
trigger  is  not  a  circuit  so  as  to  permit  current  to  pass  thru 
it  except  at  the  instant  the  resistance  of  the  spark  gap  is 
broken  down  by  over-flow  from  the  whole  antenna  system. 
It  ceases  to  be  a  circuit  substantially  as  soon  as  the  energy 
is  set  in  oscillation,  because  the  circuit  is  so  proportioned 
that  the  original  resistance  of  the  spark"  (gap)  "is  rapidly  re- 
gained, with  the  result  that  in  its  best  operating  condition  the 
action  of  the  trigger  is  quenched  after  one  half  of  one  oscil- 
lation ,  This  increases  to  a  maximum  of  2.5  oscillations  if  the 
transmitter  is  improperly  adjusted  or  not  in  normal  operating 
condition.  The  equilibrium  which  existed  in  the  antenna, 
before  the  trigger  action  of  the  circuit  C,  2,  Q,  X  and  W  be- 
gan, has  thus  been  disturbed  and  the  antenna  then  oscil- 
lates in  its  own  natural  period  until  the  energy  is  usefully 
dissipated  in  the  form  of  waves,  when  the  antenna  is  again 
charged  from  the  supply  circuit  and  the  process  repeated. 
During  the  stage  of  radiation,  the  antenna  is  cut  off  from 
the  source  of  supply  because  the  resistance  of  the  mercury 
valve  V  varies  inversely  with  the  current  flowing  thru  it. 
When  the  pressure  between  the  terminals  of  the  condenser, 
C,  equals  the  charging  pressure  no  more  current  will  flow 
into  it.  As  this  point  is  reached  the  resistance  of  the  mer- 

118 


PLATE  XII.     Front  View,  o.s-kw.  Transmitter  (Simpson  Type). 
Kilbourne  &  Clark  Mfg.  Co. 


PLATE  XIII.     Side  View,  o.5-kw.  Transmitter  (Simpson  Type). 
Kilbourne  &  Clark  Mfg.  Co. 


KILBOURNE  AND   CLARK  SYSTEM.       [201 

cury  valve  is  increased.  Consequently,  when  the  current 
is  at  a  minimum  the  resistance  of  the  valve  is  at  a  maxi- 
mum. The  converting  trigger"  (circuit)  "is  so  proportioned 
in  its  relation  to  the  mercury  valve  7,  the  patented  Simpson 
spark  gaps  Q,  the  proportion  of  capacity,  inductance  and 
resistance"  (see  paragraphs  85  and  86),  uand  the  careful 
adjustment  of  the  leads  of  the  spark  gaps  to  the  antenna 
at  the  nodes  of  potential"  (see  Chapter  Nine)  "that  under 
normal  conditions  it  will  go  out  of  action  instantly  after 
fulfilling  its  function  of  rendering  oscillatory  the  static  en- 
ergy in  the  condenser.  Any  continued  action  on  the  part 
of  the  trigger  thereafter  would  cause  energy  which  nor- 
mally ought  to  be  employed  in  antenna  radiation,  to  be  dis- 
sipated in  the  spark  gap  resistance." 

201.  The  Simpson  spark  gaps  noted  above  are  quenched 
gaps  of  unique  design,  consisting  of  two  brass  caps  screwed 
together,  on  the  inner  surfaces  of  which  are  provided  heavy 
silver  sparking  areas.  (See  paragraph  113.)  Since  both 
caps  are  electrically  connected,  being  screwed  together,  it 
is  necessary  to  insulate  one  of  the  silver  sparking  surfaces 
from  the  brass  cap  on  which  it  is  mounted.  This  is  done  by 
an  insulating  material  termed  lavite,  a  volcanic  compound 
of  excellent  insulating  qualities  and  capable  of  withstand- 
ing enormous  temperatures  without  physical  or  chemical 
disintegration.  This  type  of  gap  is  one  of  the  few  quenched 
gaps  which  is  absolutely  air-tight — thus  avoiding  the  troub- 
lesome oxides  of  combustion  which  tend  to  freely  radiate 
ions  as  well  as  to  fuse  the  sparking  surfaces — and  which 
suffers  no  insulation  troubles.  It  has  the  one  disadvan- 
tage, however,  of  not  having  its  brass  and  silver  faces 
on  the  same  line,  so  that  when  it  is  necessary  to  clean 
the  silver  surface  with  an  abrasive,  after  long  use,  the  dis- 
tance between  the  sparking  surfaces  is  increased.  If  a 
10  119 


202]       ELEMENTS   OF  RADIOTELEGRAPH?. 

means  could  be  provided  for  removing  as  much  of  the 
brass  holders  as  the  silver,  thus  keeping  the  distance  be- 
tween the  sparking  surfaces  uniform,  the  efficiency  of  this 
type  of  gap  would  be  increased,  for,  as  we  have  noted  in 
paragraphs  103  and  104,  an  extremely  minute  and  constant 
separation  of  the  gaps  is  necessary  for  proper  deionization. 
In  all  other  respects,  however,  the  design  of  the  gap  is  ex- 
cellent. This  type  of  gap  is  used  in  both  transmitters  of 
the  Kilbourne  &  Clark  Company. 

202.  From  the  description  of  these  two  types  of  trans- 
mitters, it  will  be  noted  that  they  are  both  single  circuit 
transmitters,  in  that  free  oscillations  occur  in  the  antenna 
circuit  without  sustained  electrical  connection  between  that 
circuit  and  the  supply  circuits.     As  a  consequence,  a  wave 
of  single  frequency  and  very  low  damping  is  radiated. 

203.  Sixty-cycle    current   is    commonly  used  with   the 
Thompson  transmitter  and  500-cycle  with  the  Simpson,  altho 
500  cycles  could  doubtless  be  used  with  the  former.     The 
note  with  the  latter  frequency  is  very  pleasing  and  easily 
read  thru  static  or  atmosphere  disturbances.     These  trans- 
mitters are  commonlymade  in  the  2-kilowatt  size,  altho  a  0.5- 
kilowatt  Simpson  transmitter  is  also  on  the  market.     The 
adoption  of  the  Thompson  transmitter  has  been  limited  to 
the  commercial  field,  but  the  Simpson  transmitter  has  been 
installed  by  the  Navy,  Army  and  commercial  companies. 

XXXI. 

HALLER   CUNNINGHAM   SYSTEM. 

204.  The  Haller  Cunningham  Electric  Company  of  San 
Francisco  has  developed  an  impulse  transmitter,  which, 
like  the  Thompson  transmitter,  is  based  on  the  principles 
of  Lodge.     The  diagram  and  design  of  the  gap  or  impulse 

120 


PLATE   XIV. 

Haller  Cunningham  Impulse  Excitation  Transmitter. 
(Front  View.) 


PLATE   XV. 

Haller  Cunningham  Impulse  Excitation  Transmitter. 
(Side  View.) 


FESSENDEN  SYSTEM.  [207 

circuit  is  similar  to  the  Thompson,  except  that  glass  plate 
condensers  in  oil  instead  of  the  mica  are  used.  Since  the 
capacity  of  the  condenser  is  not  required  to  be  varied  (see 
paragraph  86),  it  is  sealed  in  a  metal  container. 

205.  A  special  type  of  impulse  gap  is  employed,  incor- 
porating large  area  for  cooling  purposes  and  for  the  reduc- 
tion of  resistance  to  the  enormously  high  transient  impulse 
currents,  and  employing  a  revolving  discharger  in  order 
that  extremely  minute  separation  of  the  plates  may  be 
effected  without  danger  of  fusion.     By  accurate  machine 
work,  a  separation  of  the  order  of  0.004  inch  is  obtained. 
The  revolution  of  the  discharger  causes  such  effective  wan- 
dering of  the  spark  over  the  sparking  surface  to  be  obtained 
that  the  burrs  which  are  ordinarily  formed  on  an  impulse 
gap,  due  to  the  heavy  current,  are  absent — thus  obviating 
the  possibility  of  fusion  and  consequent  short-circuit. 

206.  Currents  of  60,  120  and  500  cycles  have  been  used 
with  this  transmitter,  the   higher  frequencies   giving  the 
most  pleasing   note.     It  has  met  with  some  adoption  in 
commercial  marine  and  coastal  installations. 

XXXII. 

FESSENDEN     SYSTEM. 

207.  The  Fessenden  transmitter,  designed  by  R.  A.  Fes- 
senden,  and  marketed  by  the  former  National  Electrical 
Signalling  Company,  employed,  as  its  chief  marks  of  dis- 
tinction, a  synchronous  rotary  spark  gap  and  compressed 
air  condensers.     500-cycle  primary  current  was  used  with 
this  transmitter,  and  by  the  use  of  the  synchronous  gap,  a 
pure  note  and  a  fairly  pure  wave — with  loose  coupling- 
were  obtained.     The  diagram  of  connections  is  similar  to 
that  in  Fig.  21,  with  the  substitution  of  the  rotary  gap  for 

121 


208]       ELEMENTS   OF  RADIOTELEGRAPHY. 


the  open  gap  shown  therein.  This  system  occasionally 
made  use  of  the  conductive  coupling  shown  in  Fig.  23. 
It  was  installed  in  many  installations  of  the  Army  and 
Navy,  one  of  the  largest  being  that  at  the  Naval  Radio 
Station  at  Radio  (Arlington),  Virginia — 100  k.w. 

XXXIII. 
MULTITONE   SYSTEM. 

208.  The  Multitone  system,  manufactured  by  the  C.  Lor- 
enz  Company  of  Berlin,  is  interesting  in  that  it  combines 
with  the  average  impulse  transmitter  a  tone  circuit  by 
means  of  which  the  note  of  the  spark  may  be  varied  with- 
out any  change  in  the  input  or  supply  current.     A  full  de- 
scription appeared  in  the  December,   1913,  issue  of  the 
Proceedings  of  the  Institute  of  Radio  Engineers,  a  brief 
summary  of  which  is  given  below. 

209.  A  diagram  is  given  in  Fig.  49.     In  addition  to  the 
primary,  secondary,  gap  or  impulse,  and  antenna  circuits, 
there  is  an  additional  circuit  of  audio  frequency,  shunted 


IH^j-^- 


Fig.  49.     Multitone  System. 

across  the  impulse  gap  Q.  In  an  impulse  transmitter,  in 
order  that  sufficient  power  may  be  placed  in  the  antenna  by 
the  hammer-blow,  whip-crack,  or  shock  method,  it  is  nec- 
essary that  the  antenna  be  impulsed  as  many  times  per 
second  as  possible.  The  only  limit  placed  on  the  number  of 

122 


MULTITONE  SYSTEM.  [209 

impulses  is  such  that  the  antenna  will  be  given  time  to 
cease  one  train  of  oscillations  before  it  is  shocked  into  an- 
other set  of  vibrations,  for  if  the  trains  of  waves  in  the 
antenna  overlap,  the  tone  of  the  signals  will  be  impaired, 
and  there  is  danger  from  reaction  between  the  gap  and 
antenna  circuits.  (If  the  antenna  circuit  be  oscillating 
while  there  is  a  current  impulse  in  the  gap  circuit,  dur- 
ing which  time  that  circuit  is  a  closed  one,  the  neces- 
sary conditions  for  the  radiation  of  a  wave  of  double 
frequency  will  obtain.  See  paragraphs  116  and  188.) 
If  several  impulses  occur  during  each  half  cycle  of 
current  in  the  secondary  circuit,  termed  partial  dis- 
charges, the  note  heard  at  the  receiver  will  be  a  smooth 
hissing  note.  This  is  because  the  impulses  are  not  reg- 
ularly spaced,  for  the  impulse  rate  will  be  greater  near 
the  top  portion  of  the  cycle,  where  the  potential  is  the 
greatest  and  the  condenser  can  be  most  often  charged, 
than  it  will  be  at  the  lower  portion.  In  order  that  a  def- 
inite pitch  may  be  given  this  note  so  as  to  give  a  greater 
auditory  effect  at  the  receiver  without,  however,  any  actual 
increase  in  energy,  the  tone  circuit,  CT  LT,  is  connected 
across  the  impulse  gap.  The  frequency  of  this  circuit  is 
an  audio  one,  500  cycles  for  instance,  and  its  oscillations 
are  impressed  or  superimposed  on  the  current  in  the  im- 
pulse circuit  with  the  consequent  production  of  a  musical 
note.  The  wave  length  for  a  frequency  of  500  cycles  is 
600,000  meters.  To  obtain  such  a  long  wave  length  in  the 
tone  circuit,  it  is  necessary  to  use  a  large  iron  cored  induc- 
tance and  a  large  capacity.  If  the  inductance  be  tapped 
and  leads  brought  out  to  a  controlling  device  similar  to  the 
keyboard  of  a  piano,  the  frequency  of  this  circuit  may  be 
changed  at  will  and  a  variety  of  musical  notes  may  be  sent 
out.  By  proper  design  or  by  ucut  and  try,"  a  range  of 
notes,  corresponding  to  the  musical  octave,  can  be  played 
at  will,  and  many  installations  of  the  Lorenz  Company,  in 

123 


210]       ELEMENTS  OF  RADIOTELEGRAPHY. 

particular — the  radio  station  on  the  Prince  of  Monaco's 
yacht  which  called  at  the  port  of  New  York  a  few  years  ago, 
have  been  so  equipped.  The  use  of  the  Multitone  System 
further  provides  a  military  advantage  of  secrecy  in  permit- 
ting the  transmission  of  characters  of  different  audio  fre- 
quency according  to  a  prearranged  schedule.  This  system, 
in  common  with  all  impulse  transmitters,  possesses  the 
advantage  of  being  a  single  circuit  transmitter  as  we  have 
previously  defined  the  term.  (The  tone  circuit  is  of  course 
an  oscillating  one,  but  it  is  not  of  radio  frequency  as  is  the 
oscillating  antenna  circuit.) 

210.  The  energy  consumed  by  the  tone  circuit  is  of  course 
furnished  by  the  gap  circuit,  and  in  the  absorption  of  this 
energy  from  the  latter,  the  tone  circuit  assists  in  the  rapid 
damping  of  the  current  in  the  gap  circuit,  just  as  the  ab- 
sorption of  energy  by  the  antenna  circuit  further  assists 
the  damping. 

211.  There  is  no  record  of  such  installations  having  been 
made  in  this  country  by  the  Lorenz  Company,  altho  the 
principle  is  applicable  to  any  impulse  system  and  as  such, 
has  been  experimented  with  by  the  Haller-Cunningham 
Company  with  fairly  satisfactory  results. 

XXXIV. 

FRENCH  POSTAL  AND   TELEGRAPH   DEPARTMENT 
SYSTEM. 

212.  The  system  of  spark  transmission  in  use  by  the 
Radio  Telegraphic  Service  of  the  Postal  and  Telegraph  De- 
partment of  France  incorporates  some  ingenious  features 
and  is  of  particular  interest  at  this  time  as  being  the  chief 
system  of  damped  wave  transmission  employed  by  our  ally 
in  the  medium  and  high  power  stations  of  her  radio  ser- 

124 


FRENCH  POSTAL  DEPARTMENT  SYSTEM.  [213 

vice.  This  system  was  designed  by  Lieutenant  Leon  Bou- 
thillon  of  the  French  Army  Engineers.  A  diagram  is  given 
in  Fig.  50.  Direct  current  of  high  potential,  obtained  by 


a 


Fig.  50.     French  Radio  Service  System. 


placing  two  or  more  generators  in  series,  is  used  to  charge 
the  condenser  C  which  is  discharged  across  the  rotary  spark 
gap  S.  This  type  of  apparatus  has  the  following  advantages 
over  the  common  alternating  current  transmitter  : 

213.  The  speed  of  the  generators  does  not  have  to  remain 
constant.  In  an  alternating  current  system,  a  variation  of 
the  alternator  speed  will  result  in  a  change  of  frequency 
and  hence  alter  the  note  of  the  spark.  Further,  a  change 
in  frequency,  will  throw  the  condenser  out  of  resonance 
with  the  charging  circuit  as  noted  in  the  preceding  chap- 
ter. Neither  is  it  necessary  that  the  speeds  of  the 
various  generators  in  series  bear  any  fixed  relation  to  each 
other.  On  the  other  hand,  if  we  desire  to  use  two  or  more 
alternators  of  high  (audio)  frequency  in  parallel  to  obtain 
an  increase  in  power,  it  is  necessary  that  they  all  be  care- 
fully synchronized,  that  is  to  say,  they  must  be  driven  at 
the  same  speed  and  in  the  same  phase  so  that  the  maxi- 
mum points  of  the  cycles  of  all  machines  will  occur  together. 
With  the  direct-current  generators  in  series,  all  of  the  ma- 
chines may  be  driven  at  different  speeds  since  no  question 

125 


214]       ELEMENTS  OF  RAD1OTELEGRAPHY. 

of  synchronism  arises  and  since  there  is  no  necessity  that 
the  potential  outputs  of  all  the  machines  be  identical. 

214.  In  addition,  the  spark  gap  does  not  have  to  be  syn- 
chronized with  the  generator.     The  rotary  spark  may  re- 
volve at  any  speed  so  as  to  obtain  the  desired  note.    While 
it  is  possible  to  obtain  a  clear  note  with  a  synchronized 
rotary  spark  gap  on  alternating  current,  the  note  in  no 
case  is  as  clear  as  with  the  direct  current  excitation,  and 
a  slight  variation  in  its  speed  will  affect  the  clearness  con- 
siderably. 

215.  No  question  of  resonance  between  the  condenser 
and  the  charging  circuits  is  involved.     (See  Chapter  Five.) 

216.  Efficiencies  of  over  90%  have  been  obtained.     The 
average  A.C.  spark  transmitter  of  best  design  rarely  aver- 
ages much  over  80%. 


126 


CHAPTER   SEVEN. 

XXXV. 

WAVE  METERS. 

217.  A  wave  meter  is  an  oscillatory  circuit,  consisting  of 
a  variable  capacity  in  series  with  a  variable   inductance, 
and  a  means  for  indicating  the  maximum  flow  of  current 
in  the  circuit.     It  is  designed  so  as  to  permit  of  a  very  fine 
and  accurate  variation  of  its  wave  length  between  the  lim- 
its of  its  range,  and  is  calibrated  or  graduated  so  that  it  is 
possible  to  know  its  wave  length  for  any  adjustment  of  its 
variable  elements.     By  placing  a  wave  meter  near  an  os- 
cillatory circuit  whose  wave  length  we  desire  to  measure, 
and  by  adjusting  the  inductance  or  capacity — either  or 
both — until  the  wave  meter  is  in  resonance  with  the  circuit 
to  be  measured,  as  determined  by  a  maximum  effect  on 
the  current  indicating  device,  we  can  determine  the  wave 
length  of  the  circuit.     With  a  potential  of  the  frequency  of 
the  current   in  this   circuit   impressed  inductively  on  the 
wave  meter,  the  maximum  current  flow  in  the  latter  is  ob- 
tained when  the  inductive  reactance  of  the  wave  meter  is 
balanced  by  its  capacity  reactance  for  that  particular  fre- 
quency.    A  condition  of  resonance  then  obtains  in  each 
circuit  and  between  both  circuits,  so  that  if  we  know  the 
wave  length  of  the  wave  meter  circuit,  we  know  that  of  the 
circuit  under  measurement  as  well — the  two  being  equal. 

218.  The  current  indicating  device  in  a  wave  meter  may 
be  one  of  two  types,  visual  or  auditory.     The  visual  type 
usually  comprises  a  small  ammeter  calibrated  either  in  frac- 
tions of  amperes  or  in  divisions  proportional  to  the  square  of 

127 


219]       ELEMENTS  OF  RAD1OTELEGRAPHY. 


the  current.  The  latter  calibration  causes  such  an  instru- 
ment to  be  termed  a  wattmeter,  for  power  in  watts  is  equal 
to  the  square  of  the  current  times  the  resistance  (see 
paragraph  19).  It  should  be  borne  in  mind,  however,  that 
such  a  wattmeter  measures  only  the  power  consumed  in 
its  own  resistance — and  shunt,  if  provided— and  not  in 
the  total  wave  meter  circuit.  A  small  galvanometer  oper- 
ated by  a  thermo-couple  (see  Chapter  Five)  may  also  be 
used  as  the  indicating  instrument,  and  its  calibration  is 
usually  proportional  to  the  square  of  the  current.  The 
auditory  method  comprises  some  form  of  detector,  a  de- 
vice for  rendering  audible  currents  of  radio  frequency  (above 
the  limit  of  audibility) — the  theory  of  whose  operation  will 

be  considered  in  a  later 
chapter — and  a  pair  of 
telephone  receivers. 
The  maximum  strength 
of  current,  that  is  to  say 
—the  resonant  condi- 
tion of  the  wave  meter, 
is  indicated  in  the  visual 
system  by  the  maxi- 
mum deflection  of  the 
needle  of  the  ammeter 
Fig.  51.  Wave  Meter.  or  wattmeter  and  in  the 

auditory  system  by  the 
greatest  response  in  the  telephone  receivers. 

219.  A  diagram  of  a  wave  meter  is  shown  in  Fig.  51.  It 
consists  of  an  inductance  in  series  with  a  milli-ammeter 
(for  measuring  thousandths  of  an  ampere)  or  wattmeter  A 
and  a  variable  condenser.  In  order  that  a  wide  range  of 
wave  lengths  may  be  measured  with  the  instrument,  it  is 
customary  to  provide  several  inductances  of  different  sizes. 
The  milli-ammeter  is  usually  of  the  same  construction  as 

128 


£ 


MAGNETIC 
GALVANOMETER 


PLATE  XVI.     Galvanometer  and  Thermo-Couple. 
General  Radio  Co. 


PLATE  XVII.     Thermo  Galvanometer  for  use  with  Decremeter. 
Weston  Electrical  Instrument  Co. 


WAVE   METERS.  [220 

the  antenna  ammeter  described  in  Chapter  Five  except 
that  it  is  designed  to  measure  very  much  smaller  cur- 
rents. It  may  either  be  of  the  hot-wire  or  the  thermo-couple 
type.  The  variable  condenser  is  a  small  air  condenser 
whose  construction  will  be  discussed  in  a  later  chapter. 
It  is  arranged  so  that  its  capacity  may  be  varied  continu- 
ously from  practically  zero  to  a  maximum  value  ranging 
from  0.001  mf.  to  0.004  mf.,  depending  upon  the  size  and 
number  of  the  plates  used  in  its  construction.  The  wave 
length  range  of  the  average  wave  meter  extends  from  about 
125  to  3,000  meters.  Others  are  supplied  ranging  from 
2,000  to  5,000  meters,  and  long  wave  length  meters  have 
30,000  meters  as  their  maximum  limit.  The  circuit  in  the 
dotted  lines  is  a  buzzer  circuit  for  setting  the  wave  meter 
into  oscillation  by  the  impact  method.  Current  from  the 
battery  is  passed  thru  the  inductance  and  a  small  buzzer. 
The  pulsating  direct  current  in  the  inductance — caused  by 
the  make  and  break  of  the  vibrator— is  thus  similar  to  that 
obtained  in  the  gap  circuit  of  an  impulse  transmitter.  The 
wave  meter  thus  becomes  a  transmitter  whose  wave  length 
can  be  varied  between  wide  limits,  and  as  such  may  be 
used  in  the  calibration  of  other  wave  meters  or  receivers. 
Its  calibration  curve  is  slightly  different  when  used  as  a 
transmitter,  due  to  the  additional  capacity  effect  of  the 
buzzer  circuit. 

220.  In  order  that  the  wave  meter  may  be  of  use,  it  must 
be  calibrated.  To  calibrate  an  instrument  is  to  graduate 
it  against  a  standard.  Thus,  a  strip  of  wood  when  gradu- 
ated in  inches  against  a  foot-rule,  becomes  itself  in  turn  a 
device  for  measuring  length.  A  wave  meter  is  calibrated 
by  using  oscillations  of  known  frequency  or  wave  length. 
Radio  laboratories  are  supplied  with  inductances  and  capaci- 
ties of  known  value,  and  by  setting  up  a  circuit  composed  of 
these  standards  and  producing  oscillations  therein — usually 

129 


220]       ELEMENTS   OF  RADIOTELEGRAPHY. 


by  the  impulse  or  shock  method— its  wave  length  may  be 
determined  by  equation  (37).     The  calibration  of  the  wave 

meter  may  be  recorded  in  two 

ways — either  by  engraving  the 
wave  lengths  of  the  meter  for 
different  condenser  settings  di- 
rectly on  the  condenser  scale  as 
shown  in  Fig.  52,  or  a  curve  of 
the  wave  lengths  for  different 
readings  of  the  condenser  in 
degrees  may  be  plotted  and 
drawn  as  in  Fig.  53.  Thus,  if 
Figt  52t  the  wave  meter,  by  the  maxi- 

mum current  indication,  demon- 
strates resonance  at  a  condenser  reading  of  75°,  a  straight 
line  is  drawn  vertically  thru  75°,  as  shown  by  the  dotted 


500 


15        30       45        GO       75        90       105 
Condenser  Reading  in  Degrees 

Fig-  53-     Wave  Meter  Calibration  Curve. 


120 


line,  until  it  intercepts  the  curve.  From  this  point,  a  line 
is  drawn  horizontally  until  it  cuts  the  vertical  wave  length 
scale.  The  wave  length  in  this  particular  case  is  seen  to 
be  400  meters,  which  is  the  wave  length  of  the  circuit  under 

130 


WAVE  METERS. 


[221 


measurement.  Where  more  than  one  coil  of  inductance  is 
supplied,  a  calibration  curve  for  each  coil  is  drawn,  or  in  the 
other  method  noted  above,  an  additional  wave  length  scale 
is  engraved  on  the  condenser. 

221.  In  place  of  the  diagram  of  Fig.  51,  the  diagrams 
shown  in  Fig.  54  are  used  when  a  detector  and  telephone 
receivers  are  employed  to  indicate  resonance  instead  of 
the  milli-ammeter  or  wattmeter.  With  this  type,  resonance 


Fig.  54.     Wave  Meters. 

is  indicated  by  the  maximum  sound  heard  in  the  telephone 
receivers.  This  method  however  is  not  as  accurate  as  the 
visual,  for  the  ear  can  only  estimate  the  maximum  current 
strength,  while  the  eye  is  assisted  by  the  scale  of  the  am- 
meter or  wattmeter  to  very  accurately  locate  the  point  of 
maximum  strength.  The  Bureau  of  Standards  states  that, 
other  things  being  equal,  with  the  connection  used  in  Fig. 
54  fa),  the  auditory  response  is  5.5  times  that  of  the  uni- 
polar connection  shown  in  (b). 
11  131 


222]       ELEMENTS   OF  RADIOTELEGRAPHY. 

XXXVI. 
DECREMETERS. 

222.  The  measurement  of  the  logarithmic  decrement  of 
the  oscillating  current  in  a  circuit  may  be  effected  by  a  wave 
meter  of  the  type  shown  in  Fig.  51.     When  a  wave  meter  is 
so  employed,  it  is  termed  a  decremeter.    The  principle  of 
its  operation  is  as  follows : 

223.  In  paragraphs  58  and  59,  we  observed  that  the  dec- 
rement or  damping  of  the  current  in  an  antenna  affected 
the  sharpness  of  tuning  of  the  receiver,  for  full  use  of  the 
principle  of  resonance  between  two  circuits  may  only  be 
made  when  the  waves  in  a  train  of  oscillations  are  of  al- 
most equal  strength.     Thus,  if  we  can  quantitatively  mea- 
sure the  sharpness  of  tuning  in  a  wave  meter  circuit,  which 
we  shall  employ  as  a  receiver,  we  shall  have  a  method  for 
the  measurement  of  logarithmic  decrement.     It  is  obvious 
that  when  the  wave  meter  (or  decremeter)  is  in  exact  res- 
onance with  the  circuit  under  measurement,  the  deflection 
of  the  indicating  needle  will  be  the  greatest.     When,  how- 
ever, we  detune  the  decremeter,  that  is  to  say — adjust  it 
to  a  tune  slightly  different  from  the  resonant  tune,  the  read- 
ing of  the  ammeter  will  be  less,  for  in  throwing  the  dec- 
remeter out  of  resonance,  we  have  increased  its  imped- 
ance, which,  in  the  resonant  condition,  is  practically  equal 
to  the  resistance  only.     The  amount  of  decrease  of  the 
current  strength,  for  a  detuning  of  a  given  quantity  will 
depend  on  the  decrement.     If,  for  a  very  slight  detuning  of 
the  decremeter,  the  decremeter  current  falls  off  consider- 
ably, whether  we  detune  to  a  longer  wave  than  the  reson- 
ant one  or  a  shorter  wave,  we  say  that  the  tuning  is  sharp 
—and  the   decrement  low.     If,  on  the  other  hand,  the 
strength  of  the  current  is  not  greatly  reduced  for  the  same 
amount  of  detuning,  we  say  that  the  tuning  is  broad — and 

132 


PLATE  XVIII.     Wave  Meter  and  Decremeter. 


DECREMETERS.  [224 

the  decrement  high.  The  amount  of  decrease  of  cur- 
rent for  a  given  detuning  from  the  resonant  condition  will 
thus  serve  as  a  measurement  of  decrement.  As  a  matter 
of  fact,  in  decrement  measurement,  it  is  rather  better  to 
measure  the  amount  of  detuning  necessary  for  a  given  de- 
crease of  current.  Accordingly,  we  find  most  decremeters 
based  on  the  principle  of  measuring  the  amount  of  detun- 
ing necessary  to  reduce  the  energy  in  the  decremeter  to 
one  half  (0.5)  of  that  in  the  resonant  condition.  This  per- 
centage is  used  when  employing  an  indicating  instrument 
whose  readings  are  proportional  to  the  square  of  the  cur- 
rent, i.e.,  a  wattmeter  or  galvanometer.  If  a  milli-am- 
meter  be  used,  whose  readings  are  the  square  root  of  those 
of  a  milli-wattmeter,  the  amount  of  detuning  for  a  seven 
tenths  (0.707)  reduction  in  current  is  measured,  since  this 
numeral  is  the  square  root  of  one  half  (0.5). 

224.  Fig.  55  illustrates  two  resonance  curves.  The  curve 
in  the  heavy  line  is  that  obtained  with  a  low  decrement  of 
the  oscillations  in  the  circuit  under  measurement.  It  will 
be  observed  that  in  plotting  this  type  of  curve,  the  readings 
of  the  wattmeter,  the  current  squared — 72,  are  used  for  the 
vertical  values,  called  the  ordinates.  The  wave  length  set- 
tings of  the  decremeter  are  plotted  horizontally,  repre- 
sented by  X,  and  are  called  the  abscissas.  In  operating 
the  decremeter,  it  is  placed  near  the  antenna  circuit,  with 
whose  decrement  we  are  particularly  concerned,  and  the 
condenser  capacity  varied  until  the  maximum  deflection  of 
the  wattmeter  is  obtained.  This  may  be  8.0,  let  us  say. 
The  coupling  between  the  decremeter  and  the  antenna  is 
then  increased  until  the  wattmeter  gives  a  reading  the 
full  length  of  the  scale  in  order  to  obtain  greater  accuracy. 
The  wave  length  of  the  decremeter  is  then  slightly  in- 
creased beyond  that  at  the  resonant  point  and  the  read- 
ing of  the  wattmeter,  which  will  be  less  than  that  at  the 

133 


224]       ELEMENTS   OF  RADIOTELEGRAPHY. 

resonant  point,  is  noted  and  marked  with  a  small  cross  on 
the  cross-section  or  graph  paper  used  in  this  work.  Several 
additional  readings  above  the  resonant  wave  length  are 
taken  and  recorded.  The  decremeter  is  then  detuned 
below  the  resonant  point  and  the  wattmeter  readings  for 


560    570      580     590  C  600  D  610      620      630     640 

A 
Fig.  55.     Resonance  Curves. 

different  wave  length  settings  noted  and  checked.  A 
smooth  curve  connecting  all  the  crosses  is  then  drawn, 
giving  the  complete  resonance  curve  shown.  The  lower 
the  decrement,  i.e.,  the  sharper  the  tuning,  the  greater  will 
be  the  decrease  of  energy  in  the  decremeter  for  a  given 
detuning  from  the  resonant  point,  and  consequently  the 
narrower  the  curve.  The  points  A  and  B  on  the  curve 
are  each  one  half  of  the  total  height  of  the  curve,  that  is  to 
say,  they  are  those  readings  of  the  wattmeter  which  are  one 
half  its  reading  at  the  resonant  point.  In  the  preceding 
paragraph,  we  observed  that  the  amount  of  detuning  which 
reduced  the  wattmeter  reading  one  half  may  be  used  to 
measure  the  decrement  so  that  the  distance  from  A  to  B, 

134 


DECREMETERS.  [227 

measured  in  meters  on  the  abscissae  scale,  is  thus  used  in 
this  calculation.  A  resonance  curve  is  thus  an  indication 
of  the  conditions  obtaining  at  the  receiving  station.  The 
amount  of  energy  represented  by  the  peak  of  the  curve 
corresponds  to  the  strength  of  the  signals  at  the  receiver 
when  tuned  to  the  transmitter's  wave.  The  more  quickly 
the  curve  drops  away  on  either  side  of  this  peak,  the  more 
quickly  will  signals  fade  out  when  the  receiver  is  slightly 
detuned  from  the  transmitter's  tune  and  the  more  easily 
will  interference  from  that  station  be  eliminated. 

225.  When  the  decrement  to  be  measured  is  high  and  the 
tuning  broad,  the  wattmeter  reading  does  not  decrease  as 
rapidly  for  the  same  detuning  as  in  the  measurement  of 
low  decrement.     The  resonance  curve  of  the  current  in  an 
antenna  of  high  decrement  is  shown  in  the  same  figure  in 
the  dotted  line.     The  distance  between  A'  and  E'  is  seen 
to  be  much  greater  than  that  between  A  and  B  and  the 
difference  in  decrements  is  thus  determined. 

226.  The  width  of  the  resonance  curve  also  depends  on 
the  decrement  of  the  measuring  circuit,  the  decremeter. 
The  value  of  this  decrement  is  given  by  the  formula  of 
equation  (32).     This  decrement  is  ascertained  in  the  cali- 
bration of  the  decremeter  and  the  value  deducted  from  the 
total  decrement  obtained  in  the  measurement. 

227.  This  measurement  of  decrement  by  detuning  is 
termed  the  reactance-variation  method,  since  in  the  de- 
tuning of   the  decremeter  we  are  varying   its  reactance 
which  is  nil  in  the  resonant  condition.     The  equation  used 
is  given  below: 

(47) 


•\!      V2-/r' 

where  o  is  the  decrement  of  the  circuit  under  measurement, 

135 


228]       ELEMENTS   OF  RADIOTELEGRAPHY. 

5i  is  the  decrement  of  the  decremeter  (see  paragraph  226), 
Xr  is  the  wave  length  of  the  circuit  under  measurement — 
the  resonant  wave  length  of  the  decremeter,  Xi  is  that  wave 
length  to  which  the  decremeter  is  detuned  to  give  a  watt- 
meter reading  of  7r,  and  7r2  is  the  wattmeter  reading  at 
resonance.  If  we  should  detune  the  circuit  so  that  7i2  is 
one  half  of  7r2,  the  expression  under  the  radical  sign  be- 
comes 

0.5  /OS  _ 

1.0  -  0.5    "  ^0.5 

Since  this  cancels  the  expression  under  the  radical  sign 
and  simplifies  the  formula,  it  is  explanatory  of  the  state- 
ment made  in  paragraph  223  concerning  the  detuning  of 
the  decremeter  such  that  the  wattmeter  reading  is  one  half 
of  that  at  resonance. 

228.  If  we  use  this  method  in  the  plotting  of  a  resonance 
curve  of  the  current  in  the  antenna  of  a  Poulsen  arc  trans- 
mitter, the  waves  of  which  are  undamped  with  consequent 
zero  decrement,  the  decrement  obtained  will  be  that  of 
the  decremeter,  for  d  of  equation  (47)  is  zero  in  this  case. 
This  method  is  used  in  the  determination  of  the  decre- 
meter decrement.     (See  paragraph  226.) 

229.  In  Fig.  55,  on  the  resonance  curve  of  lower  decre- 
ment, D  or  C  represents  Xi,  Xr  is  600  meters,  E  is  7r2  or  9.8, 
and  A  or  B  is  7i2  or  4.9.     Using  C  =  594  meters,  the  total 
decrement  according  to  equation  (47)  would  be 

600  -  594 
6  +  61  =2w  -  =  0.0634. 


230.  Equation  (47)  gives  the  decrement  as  measured 
on  one  half  of  the  resonance  curve,  and  this  is  fairly  ac- 
curate since  the  curve  is  practically  symmetrical.  That  is 
to  say,  the  distance  from  A  to  600  is  practically  that  from 

136 


DECREMETERS. 


[231 


600  to  B.  With  a  curve  of  the  shape  shown  in  Fig.  56, 
however,  the  distance  from  A  to  600  is  not  that  from  600 
to  B,  so  that  two  measurements  should  be  made  employ- 


Fig.  56. 

ing  each  half  of  the  curve,  and  the  average  of  the  results 
taken.  This  operation  may  be  combined  by  the  use  of  the 
following  formula. 

(48) 


A2 


It  is  of  course  understood  that  the  same  method  for  the  re- 
duction of  the  expression  under  the  radical  sign  to  unity  is 
employed.  In  the  above  formula,  X2  is  that  wave  length 
longer  than  the  resonant  tune  which  will  reduce  the  watt- 
meter reading  to  one  half  of  that  at  resonance,  and  Xi  is 
that  wave  length  shorter  than  the  resonant  wave  which  will 
similarly  reduce  the  wattmeter  reading. 

231.  The  values  of  the  decremeter  condenser  capacity 
instead  of  wave  length  may  be  used,  when  the  formula 
becomes 

C2  -  C, 

5  +  Sl  =  T  --? -1.  (49) 

c/2  ~r  1/1 

137 


232]       ELEMENTS   OF  RADIOTELEGRAPHY. 

In  a  variable  condenser  of  average  type — using  semi-circ- 
ular plates — to  be  described  later,  the  capacity,  except  at 
the  very  lowest  values,  is  proportional  to  the  scale  reading 
in  degrees,  so  that  these  readings  may  be  substituted  in 
equation  (49)  for  determining  the  decrement  without  know- 
ing the  condenser  capacity. 

232.  This   measurement   of   decrement   is   termed   the 
Bjerknes  method,  and  is  accurate  when  the  coupling  be- 
tween the  decremeter  and  the  circuit  under  measurement 
is  not  too  great  and  when  the  damping  of  the  current  is  not 
too  high.     In  the  measurement  of  the  current  in  the  antenna 
of  the  modern  transmitter,  these  conditions  can  be  fulfilled. 

233.  The  Bjerknes  principle  of  decrement  measurement 
is  employed  in  the  Kolster  decremeter  (invented  by  Phys- 
icist F.  A.  Kolster  of  the  Bureau  of  Standards),  a  direct  read- 
ing decremeter  (i.e.,  involving  no  computations)  which  has 
been  adopted  by  all  the  radio  branches  of  the  Government. 
By  making  the  plates  of  the  variable  condenser  of  special 
shape,  the  capacity  varies  according  to  a  geometric  pro- 
gression instead  of  the  straight  line  progression  of  the 
average  condenser.     (See  paragraph  231.)     This  enables  a 
revolving  scale,  geared  to  the  revolving  plates  of  the  con- 
denser, to  be  evenly  graduated  so  as  to  measure  the  log- 
arithmic decrement,  5  +  <5i,  directly.     The  value  of  5i  is 
given  and  subtracted  from  the  reading  on  the  decremeter 
scale.     In  its  operation,  the  condenser  capacity  is  varied 
until  resonance  is  obtained.     The  wattmeter  reading  is 
noted  and  the  decremeter  detuned  until  the  wattmeter 
reading  drops  to  one  half  of  its  value  at  resonance.     The 
wave  length  of  the  decremeter  is  now  Xi  of  the  formulae 
above.     The  decremeter  scale  is  then  set  at  zero,  and  the 
condenser  capacity  varied  so  as  to  change  the  wave  length 
from  \i  to  Xr  and  thence  to  X2.      A  complete  excursion  of  the 

138 


PLATE  XX.    Wave  Meter  and  Decremeter. 


ADJUSTMENT   OF   A    TRANSMITTER.      [236 

resonance  curve  from  Xi  to  \->  has  thus  been  made,  up  the 
resonance  curve  from  the  half-way  point  to  the  peak  and 
down  on  the  other  side  to  the  other  half-way  point.  The 
reading  of  the  decremeter  scale  at  X2  gives  the  value  of  d 
+  Si.  This  type  of  decremeter  is  made  so  as  to  be  port- 
able. It  was  originally  designed  for  the  radio  inspection 
service  of  the  Department  of  Commerce. 

234.  A  type  of  decremeter,  designed  by  J.  A.  Fleming, 
and  used  by  the  Marconi  Company,  is  operated  on  the  prin- 
ciple of  resistance-variation,  rather  than  reactance,  for  the 
measurement  of  decrement,  and  while  quite  accurate,  it 
involves  several  minutes  of  computation,  the  necessity  for 
which  is  obviated  in  the  Kolster  instrument.     Both  types 
are  in  use  by  the  Navy. 

235.  While  the  type  of  wave  meter  shown  in  Fig.  51 
may  be  used  to  measure  decrement  as  well  as  wave  length, 
if  the  decrement  of  the  meter  be  known,  the  types  in  Fig. 
54  may  be  used  only  for  the  determination  of  wave  length, 
since  with  the  detector  and  telephones  it  is  only  possible 
to  estimate  relative  strengths  of  currents  in  the  meter  in- 
stead of  making  an  accurate  quantitative  measurement  of 
them. 

XXXVII. 

ADJUSTMENT   OF  A  MODERN  TRANSMITTER. 

236.  In  the  adjustment  and  tuning  of  a  transmitter,  the 
decremeter  and  wave  meter  play  an  important  part.     The 
procedure  is  outlined  below :  Let  it  be  required  to  tune  a 
quenched  gap  transmitter  to  600  meters.     It  will  be  as- 
sumed that  the  manufacturer  of  the  transmitter  has  prop- 
erly designed  the  constants  of  the  secondary  circuit  so 
that  the  circuit  will  be  approximately  in  a  state  of  resonance. 
(See  paragraph  136.) 

139 


237]       ELEMENTS   OF  RADIOTELEGRAPHY. 

237.  The  antenna  and  ground  connections  are  discon- 
nected from  the  antenna  circuit,  and  the  key  is  depressed. 
The  wave  meter  is  placed  within  a  few  feet  of  the  gap  cir- 
cuit and  the  gap  circuit  inductance  of  the  antenna  coupler 
is  varied  until  maximum  response  in  the  wave  meter — as 
indicated  by  either  visual  or  auditory  method — occurs  at 
the  600  meter  adjustment  of  the  wave  meter.     This  may 
require    several   inductance   variations    before   the   wave 
length  is  exactly  600  meters.     The  ground  and  antenna 
connections  are  now  replaced  and  the  antenna  coupler  is 
adjusted  for  what  is  estimated  to  be  medium  coupling. 
The  key  is  again  depressed  and  the  antenna  loading  in- 
ductance, and  possibly  that  forming  the  antenna  circuit 
winding  of  the  antenna  coupler,  is  varied  until  the  antenna 
ammeter  (see  paragraph  167  et  seq.)  indicates  the  greatest 
current.     This  is  of  course  indicative  of  a  state  of  reson- 
ance between  the  gap  and  antenna  circuits.     If  the  trans- 
mitter be  sharply  tuned,  i.e.,  if  the  current  in  the  antenna 
circuit  is  of  low  decrement,  a  difference  of  one  turn  of 
loading  inductance  on  either  side  of  the  resonant  point 
will  cause  a  marked  decrease  in  the  ammeter  reading. 

238.  The  coupling  between  the  two  circuits  should  now 
be  gradually  increased,  taking  care  to  retune  the  two  cir- 
cuits to  resonance  for  each  variation  of  the  coupling,  for  the 
self-induction  of  each  circuit  is  affected  by  the  mutual  in- 
duction between  them.     Thus  if  a  transmitter  be  adjusted 
to  resonance  at  a  particular  coupling  of  the  coils,  the  cir- 
cuits will  be  thrown  out  of  resonance  if  their  coupling  be 
changed,  particularly  if  it  be  increased.     As  the  coupling 
is  increased,  exercising  the  precaution  noted,  it  will  be 
found  that  the  antenna  current  is  increased  until  a  certain 
point  of  coupling  is  reached.     With  further  increase  of 
coupling,  the  antenna  current  decreases.     Such  a  degree  of 
coupling  is  called  a  critical  point  of  coupling.     It  will  be  ob- 

140 


ADJUSTMENT   OF   A    TRANSMITTER.      [240 

served  in  actual  practice  that  sometimes  two  or  more  criti- 
cal points  of  coupling  may  exist  for  a  quenched  gap  trans- 
mitter. 

239.  The  antenna  current  is  the  greatest  for  a  maximum 
degree  of  coupling,  consistent  with  pure  quenching.     Pure 
quenching  is  that  quenching  which  will  effectually  open  the 
gap  circuit  when  the  first  pulsation  in  that  circuit  is  com- 
pleted.    A  pulsation  in  a  quenched  gap  circuit  is  shown  in 
Fig.  24,  headed  "Gap."     (When  a  quenched  gap  is  not  used 
in  a  transmitter  of  this  type,  it  reverts  to  the  Marconi  trans- 
mitter, and  several  pulsations  or  beats  (see  paragraph  74) 
occur  in  each  circuit.)     If  the  coupling  between  the  gap  and 
antenna  circuits  be  made  too  great,  the  induced  E.M.F. 
across  the  terminals  of  the  gap  circuit  winding  of  the  an- 
tenna coupler,  set  up  by  the  oscillations  in  the  antenna 
circuit,  will  be  strong  enough  to  ionize  the  gap  and  to  break 
down  its  resistance  so  as  to  close  the  gap  circuit.     (This 
break-down  voltage,  induced  in  the  gap  circuit  by  the  an- 
tenna current,  is  termed  the  ignition  voltage,  since,  if  its 
magnitude  be  great  enough,  it  will  re-ignite  the  gap  into 
a  conductive  or  ionized  state.     See  following  chapter.)     If 
this  should  occur,   the  transmitter  will  then  be   of  the 
coupled  tuned  circuit  type  of  the  Marconi  1900  patent,  and 
with  such  close  coupling,  which  is  not  harmful  if  the  gap 
circuit  be  open,  two  waves  of  widely  different  length  will 
be  radiated  from  the  antenna  circuit;  it  will  not  be  oscil- 
lating purely  (see  paragraph  110),  and  the  antenna  current 
will  be  reduced.     The  coupling  should  thus  not  be  greater 
than  that  value  which  will  just  permit  of  pure  quenching. 

240.  With  all  preliminary  adjustments  of  the  transmitter 
now  made,  the  decremeter  should  be  brought  near  the 
ground  lead  of  the  antenna  circuit  and  an  exploration  of  the 
resonance  curve  made.     It  is  not  necessary  to  actually 

141 


241]       ELEMENTS   OF  RADIOTELEGRAPHY. 


plot  a  resonance  curve  on  paper  unless  a  permanent  record 
is  desired,  for  after  a  little  practice,  an  excellent  idea  of 
the  general  shape  of  the  curve  may  be  obtained  by  watch- 
ing the  rise  and  fall  of  the  wattmeter  needle  while  varying 
the  condenser  capacity  so  as  to  cover  the  range  of  wave 
lengths  from  Xi  to  X2.  For  the  average  ship  antenna  and 
with  a  wave  length  of  600  meters,  a  logarithmic  decrement 
of  from  0.03  to  0.06  should  be  obtained. 

241.  The  resonance  curve  shown  in  Fig.  57  illustrates 
the  situation  which  will  obtain  if  the  coupling  of  a  quenched 
gap  transmitter  be  made  too  close  so  as  to  obtain  but  par- 
tial quenching,  as  set  forth  in  paragraph  239.  X,  Xi  and  X2 


Fig.  57- 

shown  in  the  figure  are  those  of  equation  (40).  X  is  the 
wave  length  of  the  antenna  circuit,  at  which  the  circuit 
will  oscillate  if  the  quenching  is  perfect,  and  Xi  and  X2 
are  the  two  coupling  waves  produced  by  the  tight  coup- 
ling and  by  the  fact  that  the  quenching  is  only  partially 
pure.  The  obvious  remedy  is  to  weaken  the  coupling.  As 
soon  as  pure  quenching  is  resumed,  Xi  and  X2  will  vanish, 
and  the  energy  which  they  contained  will  be  added  to  X, 
thus  increasing  its  height. 

142 


ADJUSTMENT   OF  A    TRANSMITTER.      [243 


242.  Fig.  58  illustrates  the  type  of  resonance  curve  ob- 
tained with  different  degrees  of  coupling  with  a  rotary 
spark  gap.  The  curve  in  the  dotted  line,  with  two  humps, 
indicates  that  the  coupling  is  too  close.  As  the  coupling 
is  weakened,  following  the  principles  set  forth  in  Section 


/\2        A          A- 1 

Fig.  58. 

XI,  the  two  coupling  waves  \i  and  X2  may  be  brought  to- 
gether into  the  resultant  wave,  X.  There  is  a  slight 
quenching  action  in  this  type  of  gap  which  permits  of  closer 
coupling  than  does  the  open  type  of  gap  of  the  Marconi 
patent. 

243.  The  resonance  curves  depicted  in  Figs.  57  and  58 
show  the  fallacy  of  taking  the  ammeter  reading  as  the 
criterion  of  operation.  The  antenna  ammeter,  since  it 
works  on  the  principle  of  the  heating  effect  of  the  total  cur- 
rent in  the  antenna  (i.e.,  of  all  frequencies),  does  not  give 
an  indication  of  the  current  strength  at  one  particular  os- 
cillation frequency  of  the  antenna.  Instead,  it  records  the 
current  of  all  the  frequencies  at  which  it  may  be  vibrating. 
Its  reading  is  thus  proportional  to  the  area  of  the  resonance 
curves  shown,  being  a  summation  or  addition  of  the  current 
at  each  wave  length  from  the  lower  to  the  upper  wave 
12  143 


244]       ELEMENTS   OF  RADIOTELEGRAPHY. 

length  end  of  the  curve.  At  the  receiver,  however,  for 
whose  maximum  response  and  sharpness  of  tuning  we  are 
working,  the  apparatus  can  be  tuned  to  but  one  wave 
length.  The  receiver  cannot  be  adjusted  so  as  to  register 
the  summation  of  currents  of  several  frequencies  as  does 
the  antenna  ammeter.  The  decremeter,  however,  is  a 
tuned  device,  and  by  its  use  the  relative  amount  of  current 
in  the  antenna  for  each  frequency  at  which  it  may  be  oscil- 
lating can  be  ascertained.  The  antenna  current,  as  regis- 
tered by  the  aerial  ammeter,  being  proportional  to  the  area 
of  the  resonance  curve,  is  thus  not  a  true  indication  of  the 
state  of  affairs  at  the  receiver.  The  antenna  ammeter  will 
often  give  a  greater  reading  when  two  coupling  waves  are 
present  (see  Fig.  58)  than  it  will  when  these  two  waves 
have  been  brought  together  to  form  one  sharp  wave.  Since 
the  area  of  a  resonance  curve  depends  upon  the  decrement 
of  the  antenna  current,  as  we  have  previously  seen,  differ- 
ences in  the  antenna  ammeter  reading  are  only  valuable 
as  indicative  of  transmitter  performance  when  the  decre- 
ment of  the  antenna  current  is  constant. 

244.  Some  interesting  results  obtained  with  an  impulse 
transmitter  are  shown  in  Fig.  59.  As  we  have  previously 
seen,  since  the  gap  or  impulse  circuit  of  this  type  of  trans- 
mitter is  non-oscillatory,  there  is  no  necessity  for  tuning 
the  gap  and  antenna  circuits  to  resonance.  The  gap  cir- 
cuit accordingly  may  be  adjusted  to  a  time  period  corre- 
sponding to  a  wave  length  of  700  meters,  with  the  antenna 
at  600  meters.  If  the  resistance  of  the  gap  should  be 
reduced  so  that  it  no  longer  damps  the  current  in  the  gap 
circuit  to  a  single  half  swing  or  oscillation,  but  permits 
several  swings  of  current  to  take  place,  two  waves  will 
occur  in  the  antenna.  This  condition  is  shown  in  the  figure 
noted.  Such  a  state  of  affairs  in  an  impulse  transmitter 

144 


ADJUSTMENT   OF  A    TRANSMITTER.      [245 

most  often  occurs  when  the  distance  between  the  sparking 
surfaces  of  the  impulse  gap  is  increased  to  too  great  a  value. 
The  necessity  of  an  extremely  minute  gap  distance  for  the 
enhancement  of  the  intensity  of  the  electric  field  and  the 
increased  absorption  of  the  ions  has  previously  been  ex- 


Fig-  59- 

plained,  and  the  detrimental  effect  of  not  complying  with 
this  provision  is  demonstrated  by  the  additional  hump  at 
700  meters,  showing  that  more  than  half  an  oscillation  is 
occurring  in  the  gap  circuit.  This  hump  will  vanish  by 
decreasing  the  sparking  distance  in  the  impulse  gaps. 

245.  Compliance  with  the  provision  that  in  a  transmitter 
radiating  waves  of  two  frequencies,  the  energy  in  the  lesser 
shall  not  exceed  10  per  cent,  of  that  in  the  greater,  may 
only  be  ascertained  by  the  use  of  the  decremeter.  Thus, 
if  the  hump  at  700  meters  in  Fig.  59  had  an  ordinate  over 
the  value  1.0,  and  the  height  of  that  at  600  meters  were 
10.0  (see  paragraph  223),  the  set  would  be  radiating  an 
impure  wave  and  would  be  violating  the  government  regu- 
lations. Similarly,  compliance  with  the  0.2  decrement  pro- 
vision can  only  be  effected  by  use  of  the  decremeter. 


145 


CHAPTER   EIGHT. 


XXXVIII. 

UNDAMPED   WAVE   TRANSMITTERS. 

246.  The  previous  chapters  have  been  given  to  a  con- 
sideration of  the  principle  of  operation  of  damped  wave 
transmitters,  i.e.,  of  those  transmitters  which  make  use  of 
the  principle  of  the  damped  oscillatory  discharge  of  a  con- 
denser— the  capacity  of  the  antenna — for  the  radiation  of 
waves.     In  the  antenna  of  the  modern  spark  transmitter, 
in  which  the  original  charge  is  allowed  to  dissipate  itself 
gradually  in  the  form  of  heat  and  radiation,  free  oscilla- 
tions— as  we  have  previously  defined  the  term — occur. 

247.  Fessenden,   Alexanderson  and   Goldschmidt  have 

developed  systems  of  undamped  wave 
•       I transmitters,  employing  forced  oscil- 
lations in  the  antenna  circuit.     Alter- 
^  nators  are  used  which  generate  al- 

ternating currents  of  radio  frequency, 
and  these  are  impressed  directly  upon 
the  antenna,  as  shown  in  Fig.  60.  It 
is  of  course  necessary  that  the  induc- 
tive and  capacity  reactances  of  the 
antenna  be  equal  in  order  that  a 
maximum  flow  of  current  in  the  an- 
tenna may  be  obtained.  In  other 
words,  the  antenna  circuit  must  be 
tuned  to  the  high  frequency  alterna- 
tor. Such  a  system  has  the  obvious 


Fig.  60. 

disadvantage  of 


requiring  an  absolutely  constant-speed 
146 


UNDAMPED    WAVE    TRANSMITTERS.      [249 

alternator,  for  a  slight  variation  of  speed  will  alter  the  fre- 
quency and  throw  the  inductive  and  capacity  reactances  of 
the  antenna  out  of  balance,  the  antenna  impedance  will  be 
increased  and  the  antenna  current  decreased.  Variations 
in  wave  length  are  also  effected  only  with  difficulty.  The 
Goldschmidt  alternator,  the  principle  of  which  is  outlined 
below,  overcomes  this  last  disadvantage  more  easily  than 
the  first  two  systems  using  radio  frequency  alternators, 
but  at  best,  its  operation  is  quite  critical. 

248.  In  Fig.  60,  no  signalling  key  is  shown.     It  is  cus- 
tomary in  forming  the  signals  to  make  and  break  the  field 
circuit  of  the  alternator  or  to  use  a  key  of  the  type  to  be 
described  in  the  latter  part  of  this  chapter.     Fig.  60  does 
not  represent  all  the  connections  for  any  of  the  systems 
noted,  but  merely  serves  to  show  the  elementary  principle 
of  operation. 

249.  The  Goldschmidt  type  of  radio  frequency  alternator 
was  in  use  for  some  time  at  the  high  power  station  at  Tuck- 
erton,  N.  J.,  at  present  operated  by  the  Navy  Department, 
and  proved  very  satisfactory.     This  system  is  based  on  the 
principle  of  frequency  changing,  that  is  to  say,  an  initial 
alternating  current  of   some  10,000   cycles   is   generated, 
which,  by  what  is  termed  the  reflection  process,  is  changed 
to  40,000  cycles — 7,500  meters.     In  the   Fessenden  and 
Alexanderson  machines,  the  radio  frequency  impressed  on 
the  antenna  circuit  is  actually  generated  at  the  terminals 
of  the  machine,  as  shown  in  Fig   60.     Such  machines  re- 
quire great  speed  and  many  field  poles,  in  order  that  such 
high  frequencies  (up  to  200,000  cycles)  may  be  obtained. 
In  the  Neuland  alternator,  by  an  ingenious  construction  of 
the  field  poles  and  armature  coils,  an  actual  moderate  speed 
may  be  multiplied  several  times  to  a  very  high  effective 
speed.     It  is  believed  that  future  development  of  the  art 

147 


250]       ELEMENTS   OF  RADIOTELEGRAPHY. 

will  bring  about  increased  adoption  of  the  radio  frequency 
generator,  which  at  present  lacks  but  few  refinements  to 
make  it  entirely  practicable. 

250.  In  the  damped  wave  transmitter,  for  each  conden- 
ser discharge  or  "  spark  "  occurring  once  during  each  alter- 
nation of  the  secondary  charging  current,  one  wave  train, 
as  shown  in  Fig.  24,  is  radiated  from  the  antenna.  With 
500  cycles,  there  are  thus  1,000  wave  trains  radiated  per 
second.  At  the  receiver,  means  are  provided  for  record- 
ing, in  the  telephone  receivers,  one  impulse  or  response  of 
the  diaphragm  per  wave  train.  Thus,  the  note  heard  in  the 
telephones  is  that  of  the  transmitter  spark,  i.e.,  1000  per 
second.  With  the  undamped  wave  transmitter,  on  the 
other  hand,  the  current  in  the  antenna  is  not  damped  out 
to  zero,  and  there  is  but  one  wave  train  radiated  for  each 
depression  of  the  key.  The  waves  in  this  train  occur,  of 
course,  at  radio  frequency  and  hence  are  inaudible  on  the 
average  receiver  designed  for  the  reception  of  damped 
wave  or  spark  signals.  Accordingly,  to  make  an  undamped 
wave  transmitter  radiate  signals  which  are  readable  on  the 
ordinary  receiver,  it  is  necessary  to  artificially  produce  wave 
trains  of  audio  frequency,  as  obtained  with  the  spark  trans- 
mitter, by  breaking  up  this  single  wave  train  into  separate 
trains.  This  is  accomplished  at  the  transmitter  by  an  in- 
strument inserted  in  the  antenna  circuit  called  a  chopper, 
a  device  similar  to  the  commutator  on  a  motor,  that  is  to 
say,  a  revolving  make  and  break  instrument  which  opens 
and  closes  the  antenna  circuit.  The  frequency  of  the  wave 
trains,  and  hence  of  the  signals  heard  at  the  receiver,  is 
equal  to  the  number  of  segments  on  the  commutator  of 
the  chopper  times  the  number  of  revolutions  per  second. 
It  is  usually  designed  to  give  about  1,000  trains  per  second. 
With  radio  sets  of  large  power,  the  flashing  and  arcing  of 

148 


THE  POULSEN  ARC   TRANSMITTER.       [252 

the  antenna  current  at  the  make  and  break  of  the  chopper 
renders  its  use  impracticable,  but  for  low  power  sets  aboard 
ship  and  at  small  coastal  installations,  it  is  quite  feasible. 
When  properly  adjusted,  the  note  is  clear  and  musical. 
The  same  result  may  be  obtained  by  inserting  the  chopper 
at  the  receiver  for  the  breaking  up  of  the  wave  train. 
When  so  used,  it  is  termed  a  tikker. 

XXXIX. 

THE   POULSEN  ARC   TRANSMITTER. 

251.  The  Poulsen  arc  was  invented  by  Valdemar  Poulsen 
of  Denmark  and  has  been  almost  exclusively  adopted  for 
continuous  or  undamped  wave  generation.     This  system 
is  in  use  at  all  the  high  power  radio  stations  of  the  Navy, 
and  on  many  naval  vessels.     Abroad,  it  is  used  by  our  allies 
as  well  as  the  enemy,  in  low,  medium  and  high  power  in- 
stallations.    In  this   country,   credit  is   due   the   Federal 
Telegraph  Company  of  San  Francisco  for  its  development. 

252.  A  diagram  of  the  essential  features  of  the  Poulsen 
arc  is  given  in  Fig.  61,  consisting  of  a  source  of  direct  cur- 


/A  A  A  X  K  V 

rr~ 

c 

•*•  — 

L   '1;   i 
t.  j 

L 
Fig.  61. 

rent,  usually  500  to  1,000  volts,  the  resistance  /?,  inductance 
coils  L,  and  the  arc  A.  The  arc  is  interposed  directly  in 
the  antenna  circuit,  the  inductance  and  capacity  of  which 

149 


253]       ELEMENTS   OF  RADIOTELEGRAPHY. 

are  represented  by  LI  and  Ci  respectively.  The  current 
flowing  in  the  direct  current  supply  circuit  is  represented 
by  /i,  that  in  the  shunted  or  antenna  circuit  by  72,  and  that 
across  the  arc  itself,  which  is  the  resultant  of  these  two 
currents,  by  /. 

253.  Ohm's  Law,  equation  (44),  which  states  that 

E  =  RI, 

does  not  hold  for  the  current  and  voltage  relations  in  the 
arc.  Instead,  the  equation  for  the  potential  across  the  arc 
is  given  by  , 

E  =  a  +  - ,  (50) 

where  a  and  b  are  constants,  the  numerical  values  of 
which  need  not  here  be  considered.  Since  /  occurs  in  the 

denominator  of  the  single  frac- 
tion of  the  equation,  the  voltage 
becomes  less  as  the  current  is 
increased,  and  conversely  for 
very  small  currents,  it  rises  to 
very  large  values. 

254.  Fig.  62  graphically  illus- 
trates the   difference  between 
the  two  equations.    The  voltage 
Fi    62  curve  for  equation  (44),  being 

directly  proportional  to  the  cur- 
rent, is  a  straight  line — since  the  variable  E  increases  just 
as  rapidly  as  the  variable  /.  The  potential  curve  for  equa- 
tion (50),  on  the  other  hand,  being  dependent  on  the  in- 
verse relation  of  the  current — since  I  is  in  the  denominator 
of  the  fraction — is  of  the  shape  shown,  and  is  termed  an 
equilateral  hyperbola. 

255.  For  extremely  small  currents,  particularly  when  the 
current  is  nil,  /  =  0,  the  formula  of  equation  (50)  is  not 

150 


PLATE   XXI. 

Power  and  Arc  Control  Switchboard. 
Federal  Telegraph  Co. 


PLATE   XXII.     loo-kw.  Poulsen  Arc  Converter. 


THE  POULSEN  ARC    TRANSMITTER.       [257 

strictly  applicable,  for  if  it  were,  the  potential  would  be 
given  by  the  equation 

E  =  a  +  oo 

=      00, 

where  the  symbol  *>  represents  infinity.  (Any  number 
divided  by  zero  equals  infinity,  for  as  the  denominator  of  a 
fraction  is  made  smaller  and  smaller,  the  value  of  the 
fraction  increases.  With  a  zero  denominator,  the  smallest 
number  to  which  it  is  possible  to  reduce  it,  the  value  of  the 
fraction  is  increased  to  an  infinite  amount.)  Instead  of 
rising  to  an  infinite  value,  the  voltage  rises  to  a  value  de- 
termined by  the  resistance  of  the  arc  gap,  such  that  it  is 
sufficient  to  re-ignite  it.  (See  preceding  chapter.)  This 
value  of  potential  is  termed  the  ignition  voltage. 

256.  When  the  electrodes,  formed  of  carbon  and  copper, 
are  brought  into  contact  with  each  other  and  then  separ- 
ated, the  spark  occurring  at  the  breaking  of  the  circuit 
volatilizes  the  carbon  and  it  becomes  incandescent.     Ions 
are  accordingly  produced  (see  paragraph  99)  which  form 
the  conducting  medium  for  the  flow  of  current  across  the 
gap  in  the  form  of  a  flame  or  arc.     (This  arc  flame  between 
two  electrodes  is  used  for  illumination  purposes  in  the  fam- 
iliar arc  light.)     The  number  of  ions  and  their  rate  of  dif- 
fusion increase  with  the  heating  of  the  electrodes  so  that 
the  resistance  to  the  current  quickly  becomes  very    ow. 
The  current  accordingly  increases. 

257.  Let  us  assume  that  thru  some  agency  the  space 
between  the  electodes  is  very  quickly  deionized,  increas- 
ing the  resistance  of  the  gap  so  that  current  no  longer 
flows  across  it.     From  equation  (50),  it  follows  that  with 
zero  current  across  the  arc  gap,  the  potential  E  rises  to  a 
large  value.     This  produces  a  high  potential  across  the 

151 


257]       ELEMENTS   OF  RADIOTELEGRAPHY. 

condenser  Ci  of  Fig.  61.  When  the  condenser  becomes 
fully  charged,  it  discharges  thru  the  inductance  LI.  Ignor- 
ing the  generator  potential,  the  resultant  potential  across 
the  arc  is  thus  the  difference  between  the  condenser  volt- 
age and  the  induced  counter  E.M.F.  of  the  inductance 
LI,  or 

E!  =  Ec  -  EL,  (51) 

where  £/  is  the  ignition  voltage,  Ec  is  the  potential  to  which 
the  condenser  has  become  charged,  and  EL  is  the  potential 
existing  across  the  inductance  LI.  The  arc  is  now  "struck" 
or  ignited  by  this  potential,  and  a  current,  /,  flows  across 
the  arc.  This  current  is  composed  of  direct  current  from 
the  generator  and  that  of  the  condenser  discharge,  or 

/  =  /i  +  /2,  (52) 

where  A  is  the  direct  current  and  72  is  the  condenser  dis- 
charge current.  When  the  condenser,  or  the  antenna  cap- 
acity which  it  represents  (see  paragraph  252),  has  become 
fully  charged  in  the  opposite  sense  (see  paragraph  26),  it 
discharges  again  in  the  opposite  direction.  At  this  re- 
versal of  the  current  72  in  the  antenna  circuit,  CiLiA,  it  is 
now  flowing  counter  to  /i,  so  that 

/  =  /i  -  /2.  (53) 

As  72  increases  in  magnitude  after  reversing,  approaching 
a  value  comparable  to  that  of  the  direct  current  /i  across 
the  arc,  the  value  of  /  will  be  so  reduced  as  to  permit  the 
arc  to  be  extinguished  by  the  magnetic  field.  The  poten- 
tial existing  across  the  arc  at  this  instant  is  termed  the 
extinction  voltage  as  distinguished  from  that  at  ignition. 
It  is  less  than  the  ignition  voltage  since  the  deionization  of 
the  arc  at  extinction  is  not  as  complete  as  it  is  just  prior  to 
reignition.  (For  the  purpose  of  simplicity  in  the  formulae 

152 


THE   POULSEN  ARC   TRANSMITTER.       [258 

above,  altho  such  procedure  entails  some  loss  of  accuracy, 
no  distinction  is  made  between  the  symbols  for  the  instan- 
taneous, or  transient,  and  the  effective  values  of  the  various 
currents  and  potentials.)  ' 

258.  Fig.  63  shows  the  voltage  and  current  curves  across 
the  arc.  From  equation  (50),  we  should  expect  to  find  ris- 
ing voltage  values  occurring  for  decreasing  current  values 
and  vice  versa,  as  shown.  The  time  period  of  the  arc,  T, 


Fig.  63. 

during  which  time  the  current  across  the  arc  makes  one 
complete  cycle,  is  divided  into  two  periods,  the  discharging 
period  (of  the  condenser),  7\ — when  the  arc  is  burning, 
and  the  charging  period,  T2 — when  the  arc  is  extinguished 
and  the  condenser  is  charging.  The  potential  peak  at  the 
beginning  of  the  period  T2  (the  lower  hump  to  the  left)  is 
the  extinction  voltage,  and  that  at  the  end  of  the  charging 

153 


259]       ELEMENTS   OF  RADIOTELEGRAPHY. 

period  (the  lower  hump  to  the  right)  is  the  ignition  voltage, 
as  defined  in  the  preceding  paragraph.  The  drop  in  poten- 
tial from  the  extinction  peak,  i.e.,  after  the  arc  has  become 
extinguished,  is  due  to  the  residual  potential  of  the  con- 
denser which  is  counter  to  that  of  the  D.C.  generator. 
However,  as  the  condenser  now  commences  to  be  charged 
again  by  the  D.C.  generator  and  its  potential  rises,  so  also 
does  that  across  the  arc,  until  the  ignition  voltage  is  reached. 
The  ignition  potential  is  not  very  much  greater  than  the 
extinction  voltage  because,  while  the  deionization  at  the 
end  of  the  charging  period  is  much  greater  than  at  the  be- 
ginning and  greatly  increased  potential  could  be  reasonably 
expected,  the  length  of  the  arc  is  less  at  the  beginning  of 
the  discharging  period  than  at  the  end  when  it  is  blown  out 
in  a  protracted  fan  by  the  magnetic  field  to  be  discussed 
later.  Hence,  with  the  reduced  arc  length,  and  consequent 
lowered  resistance,  a  lower  potential  may  be  used,  which 
compensates  somewhat  for  the  high  resistance  of  the  greater 
degree  of  deionization. 

259.  From  Fig.  63,  it  will  be  observed  that  the  current 
across  the  arc  may  be  considered  as  being  an  undamped 
alternating  current,  altho  the  current  7  does  not  actually 
reverse.  /  is  composed  of  the  pulsating  direct  current  /i, 
interrupted  by  the  periodic  extinguishing  of  the  arc,  and 
the  undamped  alternating  current  L>  of  the  condenser  (or 
antenna  capacity)  discharge.  Thus,  the  antenna  current 
of  an  arc  transmitter  differs  from  that  of  a  spark  transmitter 
in  that  instead  of  imparting  a  given  charge  to  the  antenna 
and  allowing  it  to  slowly  dissipate  itself  in  radiation  and 
impedance,  as  is  the  case  with  the  spark,  a  fresh  charge  is 
given  the  antenna  of  the  arc  transmitter  after  each  cycle  of 
antenna  current.  The  antenna  is  accordingly  kept  con- 
stantly in  oscillation,  and  the  magnitude  of  each  oscillation 
or  cycle  is  the  same  as  that  of  the  preceding  one.  Refer- 

154 


THE   POULSEN  ARC   TRANSMITTER.       [259 

ring  again  to  Fig.  63,  it  is  significant  to  note  that  the  shape 
of  the  lower  half  of  the  cycle  (current  /),  is  not  sinusoidal. 
(The  wave  shown  in  Fig.  5  is  termed  sinuoidal,  since  the 
value  of  this  curve  at  any  instant  depends  upon  the  sine  of 
the  angle — see  paragraph  41 — of  revolution  of  the  arma- 
ture coil.)  In  dealing  with  vibrations,  either  of  a  string 
under  tension  or  of  an  alternating  current,  there  is  usually 
more  than  one  frequency  to  be  considered.  The  major 
vibration  or  frequency  is  often  coupled  with  additional 
higher  frequencies  or  overtones,  called  harmonics.  These 
harmonics  are  always  exact  multiples  of  the  fundamental 
frequency.  With  an  alternator  designed  to  give  60  cycles, 
as  given  by  equation  (14),  harmonics  may  occur  at  180, 
300,  420  cycles  and  higher  frequencies.  The  upper  har- 
monic currents  are  usually  of  very  small  magnitude,  due 
to  the  increased  reactance  of  the  average  circuit,  i.e.,  con- 
taining inductance  but  not  capacity,  to  higher  frequen- 
cies. On  the  other  hand,  in  a  circuit  which  is  not  designed 
to  be  worked  on  resonant  principles — such  as  a  power 
circuit,  it  may  happen  that  while  it  is  not  resonant  at 
the  fundamental  frequency  of  60  cycles,  its  inductance  and 
capacity  may  be  so  proportioned  as  to  make  their  respec- 
tive reactances  equal  at  the  higher  frequency  of  a  har- 
monic. This  will  give  a  large  value  to  this  particular 
harmonic  current  since  there  is  practically  no  impedance 
offered  to  it.  When  an  alternating  current  contains  har- 
monics, the  shape  of  the  curve  is  not  sinusoidal  but  con- 
tains irregularities  in  it.  If  the  curve  is  symmetrical,  that 
is  to  say — has  the  same  irregularities  in  each  alternation, 
it  is  an  indication  that  odd  harmonics  only  are  present. 
(Odd  harmonics  are  3,  5,  7,  9,  11  to  2n  +  1  times  the  fun- 
damental frequency.  Even  harmonics  are  2,  4,  6,  8,  10 
to  2n  times  the  fundamental;  where  n  is  any  integer). 
When  both  odd  and  even  harmonics  are  present,  the 

155 


260]       ELEMENTS   OF  RADIOTELEGRAPHY. 

alternations  are  unlike,  that  is  to  say,  the  upper  half 
of  the  cycle  differs  in  shape  from  the  lower  half,  as  in 
Fig.  63.  With  the  Poulsen  arc,  the  current  across  the 
arc  contains  odd  and  even  harmonics.  We  shall  find  in 
the  next  chapter  that  the  antenna  circuit  will  vibrate  at 
odd  harmonics  as  well  as  the  fundamental.  Accordingly, 
with  the  Poulsen  arc  as  a  transmitter,  a  reinforcement  of 
these  odd  harmonics  of  the  antenna,  which  lie  near  the 
harmonics  of  the  arc,  results.  The  more  nearly  the  har- 
monics of  the  arc  agree  in  frequency  with  those  of  the 
antenna,  the  greater  will  be  the  reinforcement  or  excita- 
tion of  the  latter  by  the  former.  This  is  one  disadvantage 
in  the  use  of  this  type  of  transmitter,  for  it  is  possible  to 
measure  a  very  great  number  of  harmonics  on  the  aver- 
age arc  transmitter  of  long  wave  length.  The  writer  has 
measured  as  high  as  the  63rd  harmonic  on  an  arc  trans- 
mitter and  this  is  by  no  means  uncommon  on  long  wave 
length  sets.  These  harmonics  are  harmful  in  that  while 
they  do  not  contain  sufficient  energy  to  cause  interference 
at  any  great  or  even  moderate  distances,  they  are  provoca- 
tive of  a  great  deal  of  local  interference,  both  from  them- 
selves and  from  re-radiation  from  guy  wires  which  often 
may  be  in  resonance  with  the  short  wave  lengths  of  the 
upper  harmonics. 

260.  Fig.  64  shows  the  diagram  for  the  complete  Poulsen 
transmitter.  It  will  be  observed  that  the  antenna  circuit 
replaces  the  circuit  Lid  of  Fig.  63.  Across  the  direct  cur- 
rent generator  is  placed  a  voltmeter  V  for  measuring  its  po- 
tential, and  an  ammeter  A  serves  to  measure  the  current. 
No  wattmeter  is  necessary,  since  with  the  direct  current, 
it  is  only  required  to  multiply  the  voltage  by  the  current  to 
obtain  the  power  in  watts.  (See  equation  2.)  A  resistance 
R  is  provided  to  limit  the  current  into  the  arc.  Reactance 

156 


13 


THE  POULSEN  ARC    TRANSMITTER.       [261 

or  choke  coils  L  are  supplied  for  the  same  purpose  as  the 
condensers  PC  of  Fig.  44,  i.e.,  for  the  protection  of  the 
generator  from  induced  high  potential  and  high  frequency 
currents  from  the  arc  and  antenna.  In  equation  (15)  and 
paragraph  34,  we  have  observed  that  the  reactance  of  a 


Fig.  64.     Poulsen  Arc  Transmitter. 

coil  increases  with  frequency,  and  conversely — at  low  fre- 
quencies, its  reactance  is  low.  With  direct  current,  which 
corresponds  to  zero  frequency,  the  reactance  is  thus  nil. 
These  choke  coils  are  wound  on  iron  cores,  giving  them 
high  inductance  values ;  consequently  they  offer  very  high 
reactance  to  radio  frequency  currents  from  the  antenna 
circuit  and  prevent  them  from  making  their  way  back  to 
the  generator  where  they  could  cause  damage.  On  the 
other  hand,  their  reactance — as  we  have  just  seen — is  nil 
to  direct  current  and  accordingly,  they  offer  no  impedance 
to  the  current  flowing  into  the  arc. 

261.  To  enhance  both  the  extinction  and  ignition  volt- 
ages, which  is  necessary  for  periodic  extinction  of  the  arc 
and  for  a  maximum  amplitude  of  the  radio  frequency  cur- 
rent, it  is  necessary  to  provide  adequate  means  for  the 
deionization  of  the  arc.  Several  methods  are  used,  and 
their  "modi  operandi"  will  be  discussed. 

(a)  Magnetic  Field. — Fig.  64  shows  two  powerful  electro- 

157 


261]       ELEMENTS   OF  RADIOTELEGRAPHY. 

magnets,  M,  connected  in  series,  such  that  the  magnetic 

flux  set  up  between  them  is  transverse,  or  at  right  angles, 

to  the  flow  of  the  ions  across  the  arc.     In 

I     the  discussion  on  the  quenched  spark  gap, 
we  observed  that  any  artifice  which  would 

drive  the  ions  out  of  the  sphere  of  elec- 
trical action  would  contribute  materially 
to  the  deionization  of  the  gap.  The  action 
of  the  magnetic  field  on  a  stream  of  ions  is 
as  follows :  In  Fig.  65,  assume  an  ion  to  be 
Fig.  65.  flowing  away  from  the  reader  thru  the 
page  at  right  angles  to  the  plane  of  the 
paper.  It  is  represented  by  a  dot.  A  stream  of  ions  flow- 
ing in  a  straight  line  corresponds  to  the  flow  of  current 
in  a  straight  conductor,  and  the  direction  of  the  lines  of 
magnetic  force  which  they  set  up  about  themselves  in  their 
flight  is  clockwise,  as  represented  by  the  two  heavy  arrows 
of  Fig.  65.  The  right  hand  rule  of  paragraph  27  is  modi- 
fied for  determining  the  direction  of  the  lines  of  force 
around  a  straight  conductor  such  that  if  the  thumb  be 
pointed  in  the  direction  of  the  flow  of  current,  the  curved 
fingers,  at  right  angles  to  the  thumb,  point  in  the  direc- 
tion of  the  magnetic  lines  of  force,  or  flux.  If  a  mag- 
netic field  be  set  up  transverse  to  the  flight  of  these  ions, 
as  represented  by  the  lines  from  the  north  to  the  south 
pole  of  the  magnet  in  the  figure,  the  lines  of  force  of  this 
field  will  reinforce  those  set  up  by  the  ions  on  their  right, 
since  they  lie  in  the  same  direction,  and  will  offset  or  weaken 
the  magnetic  flux  generated  by  the  ions  on  their  left — since 
they  are  in  opposite  directions.  The  resultant  magnetic 
field  to  the  right  of  the  ionic  stream  is  thus  stronger  than 
that  to  the  left  of  the  ions.  The  ions  are  accordingly  moved 
or  deflected  in  a  direction  from  the  stronger  field  to  the 
weaker  one,  as  indicated  by  the  lower  arrow,  pointing  to 

158 


THE  POULSEN  ARC    TRANSMITTER.       [261 

the  left.  In  the  arc,  whose  electrodes  are  shown  in  Fig. 
66,  the  successive  stages  of  a  stream  of  ions  across  the 
electrodes  is  shown.  Starting  at  the  bottom,  the  arc  is 
quickly  blown  up  to  the  largest  fan,  where  it  reaches  such 


cu 


Fig.  66.  Fig.  67. 

a  length  that  it  is  extinguished.  In  reality,  the  distortion 
of  the  ionic  field  is  such  that  the  arc  first  strikes  on  the  edges 
of  the  electrodes  at  the  ignition  point,  marked  /  in  Fig.  67, 
and  is  blown  out  to  the  position  marked  £,  the  extinction 
point.  The  greater  length  of  the  arc  at  extinction  than  at 
ignition  is  explanatory  of  the  fact,  noted  hi  paragraph  258, 
that  the  ignition  voltage  is  not  very  much  greater  than  the 
extinction,  even  tho  the  deionization  at  ignition  is  so  much 
greater  than  that  at  extinction  as  to  lead  us  to  expect  a 
marked  difference  between  these  two  voltages.  The  ions 
set  up  in  the  arc  gap  after  the  extinction  of  the  arc,  from 
the  incandescent  electrodes,  are  removed  during  the  charg- 
ing period  T2;  which  deionization,  and  consequent  high 
resistance,  allows  the  antenna  to  become  fully  charged. 

(b)  Cooling. — As  in  the  quenched  gap,  cooling  plays  an 
important  part  in  the  deionization  of  the  arc.  The  positive 
electrode  or  anode  is  composed  of  copper,  Cu,  and  the 
cathode  of  carbon,  C,  as  shown  in  the  various  figures. 
Copper  is  used  for  the  anode  since  it  has  the  highest  heat 
conductivity  of  any  commercial  metal,  and  the  intense 
heat  generated  at  the  point  of  arc  burning  may  readily  be 
dispersed  thruout  the  entire  electrode.  To  enhance  the 
cooling  of  the  anode,  a  stream  of  water  is  circulated  con- 
stantly thru  the  duct  shown  in  Fig.  68.  Pure  water  is  a 

159 


262]       ELEMENTS   OF  RADIOTELEGRAPHY. 

non-conductor,  so  no  leakage  occurs  by  connecting  a  water 
supply  to  the  positive  side  of  even  a  1,000- volt  generator. 

Connections  to  the  anode 
must  be  made  with  rubber 
hose  for  insulation  pur- 
poses. The  use  of  salt 

_,.     ,0  water  aboard  ship  would  of 

Fig.  68.     Arc  Anode. 

course   lead   to   disastrous 

results,  since  it  would  ground  the  anode  and  short-circuit 
the  generator.  The  cooling  of  the  cathode  is  effected  by 
revolving  it  slowly.  Since  the  arc  tends  to  burn  only  on  the 
edges  and  outer  walls  of  the  electrodes,  revolving  the  ca- 
thode causes  an  even  distribution  of  the  heat  and  by  in- 
suring wandering  of  the  arc,  the  carbon  burns  away  evenly. 

(c)  Use  of  Hydrogen. — Diffusion  of  the  ions  is  assisted 
by  the  use  of  hydrogen  gas.  (See  paragraph  102.)  The 
high  velocity  of  the  hydrogen  ion  assists  the  magnetic  field 
in  very  rapidly  deionizing  the  arc,  and  in  addition,  assists 
in  the  cooling  of  the  electrodes,  since  it  has  a  very  high  heat 
conductivity.  The  use  of  hydrogen,  by  its  properties  of 
reduction,  further  eliminates  the  possibility  of  the  forma- 
tion of  metallic  oxides  which,  when  incandescent,  are  very 
prolific  generators  of  ions. 

262.  In  practice,  the  arc  electrodes  and  the  field  poles 
are  inclosed  in  a  gas  tight  chamber,  into  which  the  hydrogen 
gas  is  admitted.  Aboard  ship,  some  liquid  hydrocarbon 
such  as  alcohol  or  ether  is  used,  but  ashore — illuminating 
gas  is  successfully  employed.  Such  gas,  however,  is  often 
very  rich  in  its  carbon  content,  resulting  in  large  carbon  de- 
posits within  the  arc  chamber  which  consequently  requires 
frequent  cleaning.  In  the  expansion  of  the  gas  at  the  reg- 
ulator at  the  terminal  of  the  supply  mains,  considerable 
water  and  oil  are  liberated.  To  absorb  this  moisture  and 

160 


THE  POVLSEN  ARC   TRANSMITTER.      [264 

prevent  it  from  entering  the  arc  chamber,  it  is  customary  to 
insert  pipes  filled  with  excelsior  and  wood  shavings,  called 
scrubbers,  between  the  regulator  and  the  arc. 

263.  The  correct  value  of  the  magnetic  field  strength  is 
quite  critical.     Accordingly,  all  well  designed  arcs  are  pro- 
vided with  means  for  controlling  the  field  strength,  usually 
a  method  for  varying  the  number  of  turns  on  the  field  cores. 
The  proper  field  strength  is  that  which  will  extinguish  the 
arc  once  for  each  period  T  of  Fig.  63.     This  requires  recur- 
ring deionizations  of  the  arc  at  radio  frequency  periods,  an 
engineering  feat  requiring  no  small  degree  of  nicety  of 
operation.     With  the  longer  wave  lengths,  and  hence  fewer 
oscillations  of  antenna  current  per  second,  more  time  is 
allowed  for  deionization  or  "scavenging"  of  the  arc.     With 
the  shorter  wave  lengths,  that  is  to  say — below   1,000 
meters,  the  time  allowed  for  this  periodic  removal  of  the 
ions  is  extremely  limited  and  consequently  increased  field 
strength  is  required.     Normal  operation  of  the  arc  cannot 
be  obtained  on  wave  lengths  much  below  1,000  meters 
for  this  reason. 

264.  The  critical  nature  of  the  field  strength  is  evidenced 
by  the  fact  that  if  it  be  too  weak,  the  arc  will  not  be  extin- 
guished at  its  normal  position,  E  in  Fig.  67,  but  at  some  po- 
sition intermediate  between  the  ignition  and  extinction 
points.     The  next  arc,  and  this  is  significant,  is  not  struck 
at  the  normal  position  at  the  edge  of  the  electrode,  7,  but 
is  ignited  at  the  point  where  it  was  extinguished.     If  the 
field  be  very  weak,  this  secondary  arc  may  be  extinguished 
at  a  point  still  in  advance  of  the  normal  extinction  position, 
and  a  third  arc  ignited  at  the  same  point  to  be  extinguished 
at  the  normal  extinction  point.     The  next  arc  is  struck  at 
the  edge  of  the  electrodes,  /.     This  ignition  of  the  secondary 
and  tertiary  arcs  at  points  which  make  the  length  of  these 

161 


265]       ELEMENTS   OF  RADIOTELEGRAPHY. 

arcs  longer  than  the  normal  value,  raises  the  ignition  volt- 
age, with  a  consequent  necessary  increase  in  the  supply 
potential  and  lowered  efficiency. 

265.  Prof.  P.  O.  Pedersen,  of  Copenhagen,  has  applied 
the  term  crater  to  that  position  on  either  electrode  at  which 
the  arc  is  burning.  It  should  not  be  confused  with  the 
state  of  concavity  with  which  it  is  usually  associated. 
Thus,  at  the  ignition  point,  the  craters  on  both  electrodes 
are  at  their  edges.  At  the  moment  of  extinction,  the  craters 
are  well  back  on  the  walls  of  the  electrodes  at  the  position 
E  of  Fig.  67.  At  the  commencement  of  the  arc,  the  crater 
moves  slowly,  but  as  the  magnetic  field  blows  the  arc 
with  increasing  velocity  into  a  fan  shape,  the  craters  move 
with  growing  rapidity  to  the  final  arc  extinction  position. 
As  the  crater  marks  the  locus  of  the  arc,  it  marks  the  loca- 
tion of  the  greatest  heat  as  well.  Where  the  greatest 
amount  of  heat  is  resident  on  the  electrode,  the  maximum 
amount  of  ionization,  with  consequent  lowest  value  of  re- 
sistance and  lowest  value  of  bridging  potential,  occurs. 
Conversely,  as  soon  as  the  crater  leaves  any  particular 
point  on  the  electrode,  the  heat  and  ionization  are  reduced, 
and  the  resistance  and  potential  are  increased.  When  the 
magnetic  field  is  too  strong,  the  arc,  with  its  accompanying 
craters  on  the  electrodes,  is  projected  with  very  great 
velocity  along  them.  This  heated  crater  on  the  cathode, 
in  leaving  with  such  rapidity  the  ignition  position  on  the 
edge  of  the  carbon,  allows  the  edge  of  the  carbon  to  cool 
very  rapidly  (relatively  speaking).  As  we  have  just  ob- 
served, the  resistance  is  increased  at  this  point  of  the  gap, 
with  such  a  rapid  accompanying  rise  in  the  ignition  poten- 
tial that  another  arc  is  struck  before  the  first  is  extinguished. 
Thus,  with  too  strong  a  field,  two  or  more  arcs  may  be 
burning  at  any  one  instant,  with  consequent  irregularity 

162 


THE  POULSEN  ARC   TRANSMITTER.      [267 

in  the  antenna  circuit  oscillations.  The  general  deioniza- 
tion  of  the  gap  at  all  times  is  so  high  with  too  great  a  field 
strength  as  to  require  an  increase  in  the  supply  potential 
to  secure  the  requisite  value  of  ignition  voltage;  resulting 
in  lowered  efficiency. 

266.  When  an  arc  is  operating  normally  the  ratio  of  the 
antenna  current  to  the  supply  or  direct  current  is  expressed 
as  follows : 

Is:IA=l:-^,  (54) 

or 

-  =  0.707,  (55) 

where  Is  is  the  direct  current  to  the  arc  as  indicated  on  the 
ammeter  in  the  D.C.  leads,  and  IA  is  the  antenna  current 
as  shown  on  the  antenna  ammeter.  The  antenna  current 
is  thus  always  seven  tenths  of  the  supply  current,  when  the 
antenna  circuit  has  been  loaded  with  sufficient  inductance 
to  place  it  at  a  wave  length  well  above  its  fundamental. 
(See  paragraph  170.) 

267.  The  period  of  one  cycle  of  the  current  across  the 
arc  is  composed  of  the  charging  and  discharging  periods, 
as  noted  in  paragraph  258  and  Fig.  63.     The  length  of  time 
of  these  two  sub-periods,  and  consequently  of  the  total 
period — 71,  obviously  depends  upon  the  separation  of  the 
electrodes  and  the  ignition  and  extinction  voltages.     Ac- 
cordingly, the  wave  length  of  the  antenna  circuit  cannot  be 
calculated  solely  by  the  formula  of  equation  (37),  but  de- 
pends as  well  upon  the  factors  noted  above.     Variations  or 
fluctuations  in  the  arc  length  and  field  strength  will  thus 
affect  the  length  of  the  radiated  wave. 

163 


268]     ELEMENTS   OF  RADIOTELEGRAPH?. 

XL. 
POULSEN  ARC  KEYS. 

268.  Returning  to  Fig.  64;  in  the  lead  to  the  antenna 
is  inserted  the  loading  coil  LI,  across  a  few  turns  of  which 
is  shunted  the  key  K.  It  is  obvious  that  the  key  cannot  be 
inserted  in  the  direct  current  source  of  supply,  for  if  that 
circuit  were  opened,  the  arc  would  break  and  it  would  be 
necessary  to  re-strike  it.  Accordingly,  all  forms  of  arc 
signalling  require  that  the  direct  current  supply  to  the  arc 
be  uninterrupted.  The  form  of  key  shown  in  Fig.  64  is 
called  the  conductive  compensation  key.  When  the  key 
is  depressed  for  signalling  purposes,  two  turns  of  the 
antenna  inductance  are  short-circuited  and  the  wave  length 
is  accordingly  reduced.  The  circuit  in  the  dotted  lines 
gives  the  diagram  for  the  inductive  compensation  key. 
When  this  key  is  depressed,  the  effect  of  the  mutual  in- 
duction between  the  two  coils  is  such  as  to  reduce  the  in- 
ductance of  the  loading  coil,  and  hence  the  wave  length  of 
the  antenna.  At  the  receiving  station,  the  operator  tunes 
to  the  shorter  wave,  which  will  only  be  heard  when  the  key 
is  closed  The  wave  radiated  when  the  key  is  up  is  termed 
the  compensation  wave.  In  this  case,  it  is  longer  than  the 
working  wave.  If  the  contacts  be  arranged  as  are  the  con- 
tacts C  and  D  of  Fig.  43,  the  two  turns  at  the  lower  end  of 
the  coil  will  be  shunted  when  the  key  is  up  and  will  be 
thrown  into  the  circuit  when  the  key  is  depressed.  With 
this  arrangement,  the  compensation  wave  is  shorter  than 
the  working  wave.  It  is  often  customary  to  provide  means 
for  using  either  arrangement,  so  that  the  compensation 
tune  may  be  made  either  longer  or  shorter  than  the  work- 
ing tune.  In  any  case,  the  operator  at  the  receiver  tunes 
his  set  to  resonance  with  the  working  tune.  If  he  adjusts 
his  receiver  so  as  to  be  in  resonance  with  the  compensation 

164 


POULSEN  ARC  KEYS.  [270 

tune,  he  will  hear  only  the  intervals  occurring  between  dots 
and  dashes.  In  order  that  their  wave  lengths  may  not  be 
too  close  together  so  as  to  confuse  the  receiving  operator, 
a  5  per  cent,  difference  in  wave  length  between  the  working 
and  compensation  tunes  is  advisable. 

269.  To  obviate  the  radiation  of  two  waves  of  different 
length,  which  use  of  the  compensation  key  necessitates, 
an  arrangement  termed  the  absorbing  or  tank  circuit  is  em- 
ployed. In  Fig.  69,  the  tank  circuit  comprises  L,  R  and  C. 


Fig.  69. 

The  arc  is  thus  alternately  switched  onto  the  antenna  or  the 
tank  circuit.  When  the  key  is  in  the  "up"  position,  the 
arc  current  is  thrown  into  the  tank  circuit,  and  when  "  down" 
—into  the  antenna.  The  capacity,  resistance  and  induc- 
tance of  the  tank  circuit  are  adjusted  until  the  current 
in  the  antenna  ammeter  A  is  constant,  no  matter  to 
which  circuit  the  arc  be  connected.  No  matter  how  closely 
the  contacts  4,  5,  and  C  are  adjusted,  it  happens,  in  the 
operation  of  the  key,  that  there  are  instants  when  the  arc 
is  connected  to  neither  oscillating  circuit — with  the  result 
that  irregularities  occur  which  spoil  signalling. 

270.  To  obviate  this  difficulty,  a  reactance  key  has  been 
designed  which  prevents  the  arc  from  being  disconnected 

165 


270]       ELEMENTS   OF  RADIOTELEGRAPHY. 

at  any  instant  from  both  circuits.  It  is  shown  in  Fig.  70. 
Two  iron  cored  transformers  are  employed,  the  secondary 
of  one  being  inserted  in  the  antenna  circuit,  and  that  of 
the  other  in  the  tank  circuit,  labelled  respectively  SA  and  ST. 
Their  respective  primaries  are  marked  PA  and  PT.  The 
principle  of  their  operation  is  as  follows:  If  an  iron  core 

(ji AP-9L0AQ J  PA 


DC 


be  surrounded  by  a  coil  of  wire  thru  which  a  current  of 
direct  current  is  passed,  lines  of  magnetic  force  will  be  set 
up  therein.  As  the  current  is  increased  thru  the  coil,  the 
number  of  lines  of  force  thru  the  core  is  increased.  After 
the  current  thru  the  coil  has  reached  a  certain  value,  how- 
ever, it  is  no  longer  possible  to  set  up  additional  lines  of 
force  within  the  core.*  We  say  that  the  core  has  become 
saturated — it  cannot  hold  any  more  lines  of  magnetic  force 
no  matter  how  much  the  magnetizing  force — the  direct 
current — be  increased.  If,  with  this  direct  current  flowing 
thru  the  coil  so  as  to  saturate  the  core,  an  additional  alter- 
nating current  be  passed  thru  it,  practically  no  reactance 
will  be  offered  to  the  latter,  for  since  the  iron  is  saturated, 
the  rising  and  falling  alternating  current  will  not  be  capable 

*  This  statement  is,  of  course,  approximate,  since  complete  satura- 
tion of  iron  cannot  be  obtained. 

166 


POULSEN  ARC  KEYS.  [271 

of  setting  up  additional  rising  and  falling  lines  of  force  to 
choke  it  back.  (See  paragraph  33.)  If  the  direct  current 
be  now  shut  off,  the  iron  will  no  longer  be  saturated  and 
the  alternating  current  will  set  up  a  rising  and  falling 
flux,  the  counter  E.M.F.  from  which  will  give  the  coil  a 
high  reactance.  Thus,  if  we  wish  to  enhance  the  flow  of 
the  alternating  current  thru  a  coil,  we  have  only  to  saturate 
the  core  with  lines  of  force  set  up  by  a  direct  current  sup- 
ply; and  to  reduce  the  flow  of  A.C.,  the  direct  current  may 
be  shut  off — giving  the  coil  its  normal  high  reactance.  In 
Fig.  70,  the  primaries  of  the  transformers  are  arranged  to 
be  alternately  excited  with  direct  current  so  as  to  saturate 
the  cores  of  the  transformers,  thus  alternately  raising  and 
lowering  the  reactance  of  both  circuits  to  the  radio  fre- 
quency current  from  the  arc.  A  key,  similar  to  that  in  Fig. 
69,  is  arranged  to  throw  direct  current  from  one  primary  to 
the  other.  When  the  key  is  up,  direct  current  is  passed 
thru  the  primary  PT  of  the  tank  circuit  transformer.  This 
lowers  the  reactance  of  the  secondary  ST,  and  since  the 
primary  PA  of  the  antenna  circuit  transformer  is  open— 
the  reactance  of  its  secondary  is  very  high.  The  high 
reactance  in  the  antenna  circuit  and  the  lowered  react- 
ance in  the  tank  cause  the  radio  current  to  be  diverted 
into  the  latter.  When  the  key  is  depressed,  direct  cur- 
rent is  passed  thru  the  primary  of  the  antenna  transformer, 
lowering  the  reactance  of  its  secondary  and  the  reverse 
action  takes  place  in  the  tank  transformer.  The  react- 
ance of  the  antenna  is  now  lowered  and  that  of  the  tank 
greatly  increased — consequently  the  radio  frequency  cur- 
rent flows  in  the  antenna  circuit  and  not  in  the  tank. 

271.  While  such  a  key  appears  very  attractive  theoret- 
ically, practically  it  develops  two  serious  faults.  It  is  very 
difficult  to  keep  the  resistance  of  the  radio  frequency  or 
secondary  windings  as  low  as  they  should  be.  This  does 

167 


272]       ELEMENTS   OF  RADIOTELEGRAPHY. 

not  matter  particularly  in  the  tank  circuit  where  efficiency 
of  operation  plays  no  part,  but  in  the  antenna,  where  max- 
imum current  strength  is  desired,  it  proves  a  serious  draw- 
back, for  the  antenna  current  may  be  reduced  as  much  as 
20  per  cent,  by  SA.  After  a  protracted  run  with  this  type 
of  key,  the  residual  magnetism  of  the  core  rises  to  such  a 
value  as  to  make  signalling  impossible.  (See  paragraph 
135.) 

272.  Of  the  various   signalling  devices,   then,  for  arc 
operation,  the  compensation  key,  either  inductive  or  con- 
ductive, appears  the  most  practical,  and  is  the  most  widely 
used. 

273.  Since  the  waves  radiated  by  a  Poulsen  arc  trans- 
mitter are  undamped,  or  practically  so,  the  tuning  at  the 
receiver  is  very  sharp.     In  fact,  if  it  were  not  for  the  decre- 
ment of  the  receiver,  signals  from  an  arc  station  would 
only  be  heard  when  the  receiver  is  in    exact  resonance 
with  the  transmitter.     The  slightest  detuning  on  either 
side  of  the  resonant  point  would  cause  the  signals  to  be 
lost  entirely. 

274.  Adequate  water  supply  for  cooling  purposes,  proper 
quantity  and  quality  of  a  hydrocarbon  gas,  proper  quality 
of  carbon  in  the  cathode,  and  correct  field  strength  are  all 
necessary  for  the  successful  operation  of  the  arc,  but  with 
these  factors  determined,  the  Poulsen  arc  is  the  most 
efficient  form  of  radio  transmitter.     It  is  interesting  to 
note  that  this  form  of  transmitter  is  a  reversion  to  the  single 
circuit  transmitter  of  the  Marconi  1896  patent,  insofar  as 
the  charging  source  is  placed  directly  in  the  antenna. 

275.  Arc  transmitters  have  been  built  in  various  sizes  for 
the  Navy,  ranging  as  follows:  5,  12,  20,  35,  60,  100,  200, 
350,  500  and  1,000  kw. 

168 


CHAPTER    NINE. 
XLI. 

ANTENNAE. 

276.  The  simple  antenna  consists  of  a  single  vertical 
wire  suspended  by  a  mast.     It  was  the  type  used  by  Mar- 
coni and  the  other  pioneers  in  the  art.     Its  oscillations  are 
similar  to  those  of  a  wire  or  string  under  tension  and  will  be 
discussed  below. 

277.  If  a  cord  be  fastened  at  the  point  A  and  held  hi  the 
hand  at  the  point  B  of  Fig.  71,  and  a  sharp  movement  be 
imparted  to  it  by  the  hand,  a  wave  will  start  at  the  point  B 
which  will  run  or  traverse  the  length  of  the  cord  to  the 
point  Ay  as  shown  in  a  of  the  figure.     Here,  the  wave  will  be 


Fig.  71. 

reflected,  and  will  return  along  the  cord  to  B  on  the  oppo- 
site side  of  the  string,  as  shown  in  b.  The  length  of  time 
in  which  a  complete  excursion  of  the  cord  is  made  by  the 
wave,  from  B  to  A  and  back  to  £,  is  termed  the  period  of 
the  cord.  (See  paragraph  64.  The  relations  are  exactly 
analogous.)  If  the  movement  imparted  to  the  cord  be  a 
slow  one,  and  timed  in  resonance  with  the  period  of  the 
cord,  the  cord  will  assume  a  vibration  as  shown  in  Fig.  72. 
14  169 


278]       ELEMENTS   OF  RADIOTELEGRAPHY. 

This  is  also  due  to  the  motion  of  a  wave  travelling  from  B 
to  A  on  one  side  of  the  string  and  reflected  back  on  the 
other.  The  frequency  of  the  hand  swings  should  be  the 
reciprocal  of  the  time  period  of  the  string  in  order  that  the 
two  may  be  in  resonance.  (See  paragraph  64.)  This 
vibration  of  the  cord  is  termed  its  fundamental,  or  first 


Fig.  72. 

harmonic.  The  curve  of  oscillation  from  A  to  B  is  seen 
to  be  one  half  of  a  cycle,  or  an  alternation.  The  wave 
length,  X,  of  this  vibration  is  thus  twice  the  length  AB. 
The  points  A  and  £,  where  no  movement  occurs— since 
the  string  is  suspended  at  these  points  and  no  move- 
ment can  take  place,  are  termed  the  nodes  of  vibration, 
and  the  point  C  of  Fig.  72 — where  the  maximum  amount 
of  swinging  occurs,  is  called  the  loop.  (The  term  anti- 
node  is  sometimes  used  for  loop.) 

278.  If  the  frequency  of  the  hand  swings  be  doubled,  a 
wave  advancing  from  B  will  meet  at  the  center  of  the  cord 
a  wave  on  the  other  side,  reflected  from  A.  Since  the  en- 


Fig.  73- 

ergy  of  the  waves  on  each  side  of  the  cord  will  be  equal  at 
this  point  and  exerted  in  opposite  directions,  the  resultant 
vibration  at  this  point  will  be  nil,  and  a  node  is  accordingly 
formed  at  C  of  Fig.  73.  A  loop  occurs  on  either  side  of  the 
cord,  half  way  between  C  and  the  points  A  and  B.  It  will 

170 


ANTENNA.  [280 

be  seen  that  nodes  are  always  formed  at  A  and  B  altho  the 
position  of  the  loop  or  loops  may  be  changed.  The  fre- 
quency of  this  vibration  is  double  that  of  Fig.  72  and  the 
wave  length  is  accordingly  reduced  one  half.  It  is  termed 


Fig.  74- 

the  second  harmonic.  Similarly,  with  the  frequency  of 
the  hand  swings  trebled,  three  loops  and  two  nodal  points 
will  occur  between  the  points  A  and  B.  Such  a  vibration, 
called  the  third  harmonic,  is  shown  in  Fig.  74. 

279.  If  a  stiff,  steel  spring  be  used  instead  of  a  cord,  and 
be  suspended  only  at  £,  where  the  hand   movement  is 
imparted  to  it,  its  vibration 

will     approximately    be    as 

shown  in  Fig.  75.     Since  A 

is  no  longer  a  point  of  sus-  Fig.  75. 

pension,   the   spring  is  free 

to  vibrate  at  that  point  and  the  loop  occurs  there,  with  a 

node  at  B.     It  will  be  noted  that  the  curve  of  oscillation 

from  B  to  A  is  one  half  an  alternation  or  one  quarter  of 

a  cycle.     The  wave  length  of  this  type  of  vibration  is  thus 

four  times  the  length  of  the  spring. 

280.  Fig.  76   shows   the   sinusoidal   curve   of  potential 
distribution  for  a  simple  vertical  antenna,  and  is  similar 
to  the  curve  of  Fig.  75  for  a  vibrating  spring.     Since  the 
lower  end  is  grounded,  and  on  account  of  the  tremendous 
capacity  of  the  earth,  it  is  not  possible  to  subject  it  to  any 
appreciable  variation  in  potential  for  the   current  values 
occurring  in  the  average  antenna.   The  earth  connection 
is  thus  similar  to  the  point  of  suspension  of  the  spring  at  B, 

171 


281]       ELEMENTS   OF  RADIOTELEGRAPHY. 


1 


The  upper  end  of  the  antenna  is  insulated  and  corresponds 
to  point  A  of  Fig.  75.  Thus  a  node  of 
potential  occurs  at  the  ground  and  a 
loop  at  the  tip  of  the  antenna.  For  this 
reason,  excellent  insulation  should  be 
provided  at  the  end  of  the  antenna, 
where  the  potential  is  highest.  Theoret- 
ically, as  noted  in  the  preceding  para- 
graph, the  wave  length  of  such  a  system 
is  four  times  its  length.  Actually,  due 

to   the   stiffness  of  the   spring,   and  in 
A1_  ,  ,.  ...      . 

the   case   of   the   antenna  —  to  its  elec- 

trical  constants,  C,  L  and  R  —  the  wave 


»• 


Flg*  76> 


length  is  slightly  more  than  this  value,  being  between  four 
and  five  times  its  length. 

281.  Fig.  77  illustrates  the  curve  of 
current  distribution  in  this  type  of  an- 
tenna. It  will  be  seen  that  conversely 
to  the  potential  distribution,  there  is 

a  loop  of  current  at  the 

ground,  and   a   node   at 

the   tip  of   the  antenna. 

For  this  reason,  large  size 

wire   or   strip  conductor 

should    be   provided  for 

the    ground    where    the 

current  is  greatest. 

282.  Fig.  78  shows  the  first  overtone  or 
third  harmonic  of  the  potential  of  a  simple 
antenna.  It  will  be  noted  that  its  wave 
length  is  one  third  that  of  the  funda- 
mental vibration.  As  will  be  seen  by 
continuing  the  potential  curve  with  the 
172 


A^T^ 


rA 

1 
\ 

\ 

\ 

\ 

V 

\I 

\ 

\ 

\ 

B 

\ 

Fig.  77. 


< 

*>* 


Fig.  78. 


ANTENNJE. 


[284 


K 


I 


T 


dotted  line— to  form  one  complete  cycle— its  wave  length 
is  4/3  the  length  of  the  antenna,  AB. 

283.  That  it  would  not  be  possible  for  the  antenna  to 

vibrate  at  the  second  harmonic,  for  the  first  overtone,  is 

shown  in  Fig.  79.     Two  potential  curves 

are  given,  illustrating  the  two  possibilities 

of  vibration.    It  will  be  seen  that  while  the 

wave  length  in  either  case  is  twice  the  length 

of  the  antenna,  or  one  half  of  that  of  Fig. 

76,  thus  making  it  the  second  harmonic, 

it  involves  the  presence  of  either  a  node 

of  potential  at  each  end  of  the  antenna,  or 

a  loop.     This  is  equivalent  to  having  the 

antenna  circuit  either  grounded  at  both 

ends   or  free   at    both    ends.     Actually, 

neither  situation  obtains,  so  that  it  is  not  possible  for  the 

antenna  to  vibrate  at  the  second  harmonic,  or  any  other 

even  harmonic.  The  grounding  of  the  antenna  at  the  lower 
end  and  its  insulation  at  the  upper,  limits 
its  vibration  to  the  fundamental  and  odd 
harmonics  only.  The  wave  lengths  of  the 
latter  are  1/3,  1/5,  1/7,  1/9,  etc.,  of  the 
fundamental. 


Fig.  79. 


284.  When  an  inductance  coil  is  inserted 
in  an  antenna,  as  is  the  case  in  all  radio 
transmitters,  a  sharp  rise  of  potential  oc- 
curs across  the  coil,  as  given  by  equation 
(45).     This  is  shown  in  Fig.  80.     From  B 
to  C,  the  potential  curve,  is  sinusoidal  in 
shape,  from  C  to  D  a  quick  rise  in  potential 
across  the  inductance  coil  L  takes  place, 
from  D  to  the  tip  of  the  antenna,  the  curve  is  again  sinu- 
soidal.    If  the  curve  from  H  to  D  be  extended  in  a  sine 

173 


Fig.  80. 


285]       ELEMENTS   OF  RADIOTELEGRAPHY. 

curve,  as  shown  in  the  dotted  line,  the  effect  of  the  in- 
ductance in  increasing  the  wave  length  will  be  shown. 
The  wave  length  will  now  be  four  times  AF  instead  of  four 
times  AB. 

285.  The  opposite  effect  of  a  condenser  connected  in  the 
circuit  is  illustrated  in  Fig.  81.  A  reversal  in  potential 
across  the  capacity  takes  place  from  C  to  D.  The  wave 
length,  instead  of  being  four  times  AB  is  now  four  times 
AE,  showing  how  the  wave  length  has  been  shortened. 
(See  paragraph  171.)  It  may  be  shown  by 
-7-  a  graphical  delineation  of  the  reactance  of  an 
/  antenna,  which  does  not  follow  the  formulae 
/  given  in  equations  (15)  and  (16)  because  of 

the  distributed  capacity  and  inductance  but 
has  a  cotangent  function  introduced  therein, 
that  the  insertion  of  either  series  inductance 
/  or  capacity  in   the   antenna  in   altering  the 

fundamental   wave   length,   also    affects  the 


Fig.  81.  frequencies  of  the  antenna  harmonics  so  that 
they  are  no  longer  integral  multiples  of  the 
fundamental  frequency.  Thus  the  frequencies  of  the  radi- 
ated harmonics  of  a  Poulsen  transmitter  (see  paragraph 
259)  are  not  exact  multiples  of  the  fundamental  frequency 
due  to  the  large  value  of  the  antenna  loading  inductance. 

286.  It  is  interesting  to  note  than  in  earthing  an  antenna, 
we  are  giving  the  resultant  circuit  the  same  wave  length 
it  would  have  if  the  antenna  were  connected  to  an  oscil- 
latory circuit  which  was  a  duplicate  of  itself.  In  other 
words,  the  earth  connection  serves  as  an  image  of  the  an- 
tenna proper.  The  wave  length  of  the  antenna  circuit,  A  to 
ground  in  the  accompanying  figures,  is  thus  twice  that  of  the 
antenna  alone.  Accordingly,  if  the  antenna  circuit  were 
opened,  the  antenna  wave  length  would  drop  to  one  half. 

174 


VARIOUS   TYPES   OF  ANTENNA.          [288 

We  have  noted  in  paragraph  171  that  the  wave  length  of  the 
antenna  circuit  may  be  reduced  by  inserting  a  capacity 
therein.  As  the  capacity  is  made  smaller  and  smaller,  its 
reactance  grows  larger  and  larger,  until  finally  when  the 
capacity  equals  zero,  its  reactance  is  infinite,  corresponding 
to  a  break  in  the  circuit.  At  this  point,  the  wave  length  has 
been  reduced  one  half  as  we  have  just  observed.  Con- 
sequently, it  is  not  possible  to  reduce  the  antenna  circuit 
wave  length  to  a  value  less  than  one  half  of  the  fundamental 
by  the  insertion  of  series  capacity. 

XLII. 
VARIOUS   TYPES    OF  ANTENNAE. 

287.  The  simple  vertical  antenna  described  above  did 
not  have  sufficient  capacity  to  serve  as  a  storage  and  radi- 
ative circuit  of  large  powers,  altho  it  did  have  the  advantage 
of  radiating  equally  well  in  all  directions.     In  order  that 
the  effective  capacity  of  the  antenna,  which  depends  upon 
the  spacing  between  the  wires  in  the  antenna  in  the  region 
of  the  potential  loop,  i.e.,  near  the  end  of  the  antenna,  and 
the  proximity  of  this  portion  of  the  antenna  to  earth,  might 
be  increased,  the  simple  vertical  antenna  of  one  wire  has 
been   superseded   by  the   types   described   below.      The 
capacity  of  the  average  ship  anten- 
na is  about  0.001  mf.,  that  of  high  A 

power  shore  stations  ranges  from    "*" 
0.014  to  0.02  mf. 

288.  Inverted  L. — As   shown  in 
Fig.  82,  this  type  takes   its   name 

from  its    shape.      The    horizontal  Fig.  82. 

portion    of    the   antenna,    AB,    is 

stretched  between  two  vertical  supports,  either  masts  or 

175 


289]       ELEMENTS   OF  RADIOTELEGRAPHY. 


towers.  It  consists  of  two  or  more  wires  strung  between 
wooden  spreader s,  as  illustrated  in  Fig.  83.  Insulators  are 
inserted  in  the  bridle,  and  frequently  in  each  wire  at  both 
ends — to  insulate  it  from  the  spreaders.  The  vertical  por- 


X 


X 


Fig.  83. 

tion  of  the  antenna  is  termed  the  lead-in.  It  may  consist 
of  one  or  more  wires.  This  type  of  antenna,  when  its 
horizontal  portion  is  many  times  longer  than  the  vertical, 
radiates  most  effectively  in  the  direction  indicated  by  the 
arrow  in  Fig.  82. 

289.  T  Type.— This  form  of  antenna  also  takes  its  name 
from  its  shape,  as  shown  in  Fig.  84.     It  differs  from  the 

inverted  L  only  in  the  position 
~~  from  which  the  lead-in  is  taken. 
Since  the  portions  AC  and  CB 
may  be  considered  as  being  in 
parallel  with  each  other,  the  in- 
ductance of  the  horizontal  por- 
tion of  the  antenna  is  less  than 
that  of  the  inverted  L  type,  with 
the  result  that  the  wave  length 
of  this  antenna — for  equal  dimen- 
sions— is  less  than  that  of  the  former.  It  is  directive 
equally  well  from  each  end,  as  indicated  by  the  arrows. 

290.  Both  of  these  types  lend  themselves  very  favorably 
for  installation  aboard  ship — their  horizontal  portions  being 

176 


Fig.  84. 


VARIOUS   TYPES   OF  ANTENNA.          [290 

suspended  between  the  masts,  and  the  lead-ins  brought 
down  to  the  operating  cabins.  Ashore,  they  are  also 
employed  with  occasional  variations,  such  as  that  shown 


Fig.  85. 

in  Fig.  85,  which  is  triangular  in  shape.  Since  the  lead-in 
is  brought  in  from  one  end,  it  may  be  considered  as  being 
similar  to  the  inverted  L ;  if  it  were  connected  at  the  center 
at  the  point  C,  as  shown  in  the  dotted  lines,  it  would  re- 
semble the  T  type.  The  Marconi  Company  in  its  high 
power  stations  has  made  extensive  use  of  the  directive 
properties  of  the  inverted  L  type,  in  both  transmitting  and 
receiving  antennae.  (See  paragraph  177.)  Such  antennae 
have  been  erected  at  a  height  of  300  feet  and  with  horizon- 
tal portions  1.25  miles  in  length.  This  type  of  construction 
gives  strong  directional  qualities  in  the  direction  noted  in 
Fig.  82.  The  high  power  radio  stations  of  the  Naval  Com- 
munication Service,  on  account  of  the  necessity  of  being 
required  to  communicate  equally  well  in  all  directions,  have 
not  been  constructed  with  a  view  toward  obtaining  marked 
directional  qualities  in  their  antennae.  Accordingly,  the 
three  high  power  Naval  radio  stations  at  San  Diego,  Cal., 

177 


291]       ELEMENTS   OF  RADIOTELEGRAPHY. 


Pearl  Harbor,  T.H.,  and  Cavite,  P.  I.,  have  been  constructed 
with  antennae  laid  out  in  a  tri- 
angular shape   at   a  height    of 
600  feet. 

291.  Fan  or  Harp  Type.— The 
fan  type  of  antenna,  shown  in 
Fig.  86,  was  used  to  some  ex- 
tent by  the  Federal  Telegraph 
Company  in  their  coastal  in- 
stallations. The  wires  compos- 
ing it  are  suspended  in  a  ver- 
tical plane  from  a  horizontal 


Fig.  86. 


support  stretched  between  two  masts. 

292.  Umbrella.—  This  type  of  antenna  is  shown  diagram- 
matically  in  Fig.  87.  From  a  single  mast,  several  wires 
(only  two  of  which  are  shown)  are  suspended  in  all  direc- 
tions. A  short  mast  for  the  terminus  of  each  radial  wire 
is  required,  or  the  wire  may 
be  connected  directly  to  some 
sort  of  ground  anchor,  with 
insulators  inserted  in  the  wire 
at  a  considerable  distance 
above  the  earth.  This  type 
of  antenna  has  very  high  ca- 
pacity (see  paragraph  287)  and 
radiates  equally  well  in  all 
directions.  It  has  been  widely 
used  by  the  Telefunken  Com- 
pany for  land  installations,  and  serves  admirably  —  in 
smaller  sizes — for  a  portable  antenna  for  use  in  the  field. 
As  such,  it  has  met  with  considerable  degree  of  adoption 
by  the  military  services  of  various  countries. 

178 


Fig.  87. 


VARIOUS   TYPES   OF  ANTENNA.          [293 

293.  Ground  Antenna.1 — A  type  of  ground  antenna  is 
shown  in  Fig.  88.  This  form  of  aerial  has  not  been  adopted 
for  use  in  any  radio  system  but  is  interesting  from  the  ex- 
perimental standpoint.  The  horizontal  portion  may  be  laid 


1%%%%%%^^ 

Fig.  88. 

along  the  surface  of  the  earth,  on  posts  a  few  feet  high,  or 
in  trenches  below  the  ground  surface.  The  ends  of  the 
horizontal  portion  have  variously  been  left  open,  grounded, 
or  connected  to  earth  thru  condensers  as  shown.  When 
the  antenna  is  suspended  on  posts  sufficiently  high  to  pre- 

1  Recent  experiments  with  the  ground  antenna  have  shown  that  if 
the  antenna  wire  be  buried  in  a  shielding,  conducting  medium  such  as 
sea  water  or  marsh  land  of  high  conductivity,  but  insulated  therefrom, 
the  effect  of  static  waves  or  strays,  which  are  practically  Hertzian 
(ungrounded)  in  nature,  is  virtually  overcome.  As  will  be  noted  later, 
wave  propagation  appears  to  consist  of  ground  currents  accompanied 
by  static  forces  or  strains  in  the  space  above  the  earth's  surface. 
When  employing  the  subterranean  antenna,  signals  are  diminished  due 
to  the  sacrifice  of  the  static  forces  of  wave  transmission,  the  conducting 
area  around  the  antenna  shielding  it  from  these  static  strains.  The 
effect  of  the  ground  component  of  the  wave  is  fully  utilized.  On  the 
other  hand,  the  Hertzian  strays — which  have  no  ground  component — 
are  completely  shielded  from  the  antenna  and  eliminated.  To  bring 
signal  strength  up  to  the  desired  audibility  in  order  to  compensate  for 
the  diminution  noted  above,  audion  amplifiers — to  be  described  later 
— are  used. 

It  may  be  stated  as  axiomatic  that  the  successful  static  or  stray  pre- 
venter must  reduce  the  ratio  of  static  to  signal  strength.  Many  so-called 
preventers  have  reduced  the  audibility  of  both  without  altering  their 
relative  strength.  Amplification  applied  to  such  a  preventer  merely 
returns  conditions  to  their  former  status. 

179 


294]       ELEMENTS   OF  RADIOTELEGRAPHY. 

vent  direct  sparking  to  earth,  it  may  be  used  for  transmis- 
sion. Ordinarily,  however,  this  type  of  antenna  has  been 
used  solely  for  purposes  of  reception,  and  as  such — dis- 
counting its  strong  directive  qualities,  indicated  by  the  ar- 
rows— has  proved  very  effective.  Unless  the  wires  are 
laid  in  soil  of  perfect  conductivity  or  sea  water,  interfer- 
ence from  static  or  atmospheric  disturbances  is  quite  as 
prevalent  as  with  the  more  common  types  of  antenna. 
An  arrangement  using  a  set  of  such  wires  laid  out  radially, 
and  having  means  for  the  reception  on  any  one  at  will, 
would  prove  effective  for  the  location  of  a  radio  station  from 
which  signals  could  be  intercepted.  The  particular  wire  on 
which  signals  were  received  with  greatest  intensity  would 
point  in  the  direction  from  which  they  were  emanating. 

294.  Loop  Antenna. — Probably  the  most  recent  type  of 
antenna  is  the  so-called  "  loop."  This  takes  its  name  from 
the  fact  that  it  is  rectangular  in  shape  and  consists  of  sev- 
eral turns  of  wire  wound  in  a  rectangular  coil  and  sus- 
pended in  a  vertical  plane  between  masts.  No  ground 
connection  is  used  with  this  type  of  antenna  in  its  simplest 
form,  the  loop  forming  part  of  the  secondary  circuit.  (See 
paragraph  335.)  Such  an  antenna  has  extremely  marked 
directional  qualities  in  the  line  of  its  vertical  plane  or  at 
right  angles  to  its  axis.  For,  when  the  loop  is  erected  so 
that  its  axis  is  pointing  in  the  direction  of  the  transmitting 
station,  the  lines  of  force  cut  opposite  sides  of  the  loop 
simultaneously  and  the  induced  currents  flowing  in  the 
same  direction  in  each  side  of  the  loop  cancel  each  other. 
When  the  loop  is  turned,  however,  so  that  its  axis  is  at 
right  angles  to  the  direction  of  the  advancing  wave,  there 
is  a  finite  interval  of  time  elapsing  between  the  induction 
of  potentials  in  the  opposite  sides.  This  results  in  currents 
which  are  not  in  phase,  and  the  strength  of  received  sig- 
nals, depending  on  the  phase  difference  between  these 

180 


TOWER   CONSTRUCTION.  [295 

opposing  currents,  will  be  a  maximum  when  the  plane  of 
the  coil  is  pointing  to  the  transmitting  station. 

Contrary  to  the  ground  antenna  described  in  the  foot- 
note on  page  179,  this  type  of  antenna  makes  use  solely  of 
the  static  component  of  the  wave  and  sacrifices  the  ground 
currents.  This  loss  of  energy  is  compensated  however, 
by  the  reduced  interference  from  stations  which  do  not  lie 
on  the  exact  bilateral  bearing  of  the  plane  of  the  loop. 
Interference  from  strays  is  also  greatly  minimized  since 
obviously  only  those  atmospheric  waves  can  be  heard  which 
originate  at  points  on,  or  near,  the  bearing  of  the  loop  plane. 
The  use  of  amplifiers  compensates  for  the  weakness  of 
signals  due  to  the  sacrifice  of  the  ground  component  of 
the  wave. 

When  a  loop  is  built  of  such  dimensions  that  it  may  be 
readily  rotated,  and  is  provided  with  the  necessary  scales 
and  other  measuring  gear,  it  serves  as  a  radio  compass  or 
pelorus  for  the  location  of  transmitting  stations.  By  means 
of  cross  bearings  taken  from  two  locations,  a  very  accurate 
"  fix  "  or  intersection  may  be  obtained.  This  type  of  radio 
direction  finder  was  invented  in  this  country  by  Physicist 
F.  A.  Kolster  of  the  Bureau  of  Standards,  and  has  been 
extensively  used  during  the  European  war. 

The  loop  antenna  is  being  widely  adopted  on  account  of 
its  marked  freedom  from  atmospheric  and  extraneous  trans- 
mitter interference  and  will  find  its  greatest  employment  at 
congested  centers  of  radio  traffic  where  separate  loops  will 
be  erected  for  each  transcontinental  and  transoceanic 
circuit. 

XLIII. 

TOWER  CONSTRUCTION. 

295.  The  simplest  form  of  vertical  suspension  for  anten- 
nae is  the  ordinary  ship's  mast,  consisting  of  lower  mast, 

181 


296]       ELEMENTS   OF  RADIOTELEGRAPHY. 

topmast  and  occasionally  a  topgallant.  Such  a  mast  re- 
quires stays,  in  which  are  inserted  insulators,  in  order  that 
their  length  may  not  make  them  resonant  with  the  radiated 
wave  from  the  antenna,  or  harmonics  thereof.  (See  para- 
graphs 259  and  283.)  Wooden  lattice  towers  have  been 
successfully  employed  by  the  Navy  Department  and  the 
Federal  Telegraph  Company.  They  have  the  advantage  of 
not  absorbing  energy  from  the  waves  radiated  from  1he 
antenna  nor  reradiating  them,  but  require  an  elaborate  sys- 
tem of  stays.  Steel  towers,  on  the  other  hand,  as  used  by 
the  Navy  Department,  may  be  built  with  large  bases  tapering 
to  the  top,  similar  to  the  Eiffel  Tower.  These  are  self- 
supporting  and  do  not  require  stays.  They  require  insula- 
tion from  the  earth,  however,  either  in  the  form  of  porcelain 
or  glass.  The  Marconi  Company  has  employed  steel  tubular 
masts,  and  the  Telefunken  Company  steel  lattice  towers, 
both  of  which  types  require  stays  and  ground  insulation. 

296.  Towers  and  masts  are  either  built  in  sectional  fash- 
ion, that  is  to  say,  built  from  the  ground  up,  or  in  slanting 
or  horizontal  positions  from  which  they  are  erected  in  one 
piece  to  a  vertical  position  with  the  use  of  jury  masts. 

297.  The  insulation  in  an  antenna,  particularly  at  the 
point  farthest  from  the  apparatus  and  where  the  potential  is 
the  highest  as  previously  noted,  is  subjected  to  great  elec- 
trical strain  and  must  be  of  the  very  highest  quality.     Porce- 
lain and  electrose  (a  commercial  compound)  are  commonly 
used,  altho  the  Marconi  Company  has  occasionally  em- 
ployed hard  rubber  where  the  power  used  is  not  too  great. 

XLIV. 
EARTH   CONNECTIONS. 

298.  Proper  and  adequate  connection  to  earth  is  a  neces- 
sary factor  for  a  successful  antenna,  particularly  for  trans- 

182 


PLATE  XXVI.     Lattice  Tower. 


PLATE   XXVII.     Steel  Tower.     (Navy  Type.) 


EARTH  CONNECTIONS.  [300 

mission  purposes  where  the  antenna  current  at  the  ground 
connection  is  large  and  the  energy  lost  in  ground  resist- 
ance may  be  excessive.  Two  types  of  earth  connection 
are  employed,  conductive  and  counterpoise.  In  the  for- 
mer, direct  contact  with  the  earth  is  made ;  in  the  latter,  a 
large  area  of  metal  is  spread  over  the  ground,  and  insulated 
therefrom,  serving  as  one  plate  of  a  very  large  condenser 
with  the  earth  as  the  other.  A  complete  analysis  of  the 
subject  has  been  made  by  Zenneck,  parts  of  which  will  be 
reviewed  below. 

299.  In  making  an  adequate  earth  connection,  the  nature 
of  the  soil  must  be  taken  into  consideration.     It  may  be 
found  that  the  soil  is  a  very  good  conductor,  or  that  the 
upper  portion  may  have  a  high  resistance  but  at  a  short 
distance  below  the  surface  there  is  conductive  water — either 
flowing  or  stagnant,  or  that  the  ground  is  a  very  poor  con- 
ductor with  no  subterranean  water  near  enough  the  surface 
to  be  effective. 

300.  Between   the   antenna   and   the   ground,   lines   of 
static  force  are  set  up.     Those  which  pass  thru  the  air 
alone  are  not  accompanied  by  a  flow  of  current,  and  no 
loss  is  occasioned  thereby.     Those,  however,  which  flow 
thru  a  conductor  set  up  currents,  and  unless  the  resistance 
to  these  currents  be  low,  energy  will  be  wasted  in  over- 
coming it.     It  is  essential,  therefore,  that  for  those  lines  of 
force  which  are  to  pass  thru  a  conductor  in  order  to  make 
the  complete  excursion  from  the  antenna  proper  to  the 
lower  terminal  of  the  ground  lead,  as  low  a  resistance  as 
possible  be  provided.     And  just  as   the  field   of  greatest 
static  strain  on  a  condenser  is  the  locus  of   the  greatest 
loss,  as  noted  in  paragraph  145,  so  the  regions  where  the 
ground  currents  are  most  closely  confined  mark  the  seat 
of  the  greatest  expenditure  of  energy  in  an  antenna  cir- 

15  183 


301]       ELEMENTS   OF  RADIOTELEGRAPHY. 


cuit.     No  harm  is  caused  if  the  static  lines  of  force  be 
concentrated  at  a  point  where  the  resist- 
ance is  infinite — as  in  the  air — because 
/         no  current  flow  can  occur  to  waste  en- 
ergy, but  should  such  points  be  present  in 

the  earth,  excessive  losses  will  take  place. 
\ 

\  301.  Thus,  if  an  antenna  be  grounded 

by  a  single  wire,  represented  by  the  dot 
Fig.  89.  in  Fig.  89,  the  density  of  the  ground  cur- 

rents about  it  will  be  very  great — as 
shown.  If,  on  the  other  hand,  this  ground  wire  be  replaced 
by  a  piece  of  sheet  metal  below  the  surface  of  the  earth, 


YM 

/     / 

/     / 

/     ''      ''" 

\      I         ! 

/       '     i 

\      > 

\      \ 
\        \ 

\               x 
\              ^^ 

Fig.  90. 

as  shown  in  Fig.  90,  the  concentration  of  the  earth  currents 
about  it  will  not  be  so  intense 
and  less  loss  will  occur. 

302.  If  a  small  (relatively 
speaking)  plate  be  buried  in 
soil  of  fair  conductivity,  the 
condition  shown  in  Fig.  91  will 
obtain.  From  the  above,  it 
will  of  course  be  obvious  that 
large  losses  will  occur  with  so 
great  a  concentration  of  the 


Fig.  91. 


earth  currents  in  the  soil  surrounding  the  plate. 

184 


On  the 


EARTH  CONNECTIONS. 


[303 


other  hand,  if  a  large  area  of  metal  sheet  be  placed  under 

the  ground,  as  in  Fig.  92,  there  will  be  no  concentration  of 

the   lines  of  force  in  the 

earth,  and  a  path  of  low  re- 

sistance will  be  provided 

them.     (There  is  obviously 

some  loss  consumed  in  tra- 

versing  the    distance   be- 

tween the   surface   of  the 

earth  and  the  buried  plate.) 

The   same    condition   will 

obtain  if  direct  connection  p. 

is  made  between  a  small 

buried  plate  and  a  stream  of  conductive,  subterranean  water. 

In  the  latter  case,  the  heavy  line  of  Fig.  92  will  represent 

the  underground  water  instead  of  the  sheet  metal. 

303.  The  use  of  so  large  an  area  of  sheet  metal,  how- 
ever, would  prove  enormously  expensive  and  accordingly 
would  not  be  practical.  In  its  place,  a  network  of  wire  is 
commonly  substituted,  usually  in  great  squares  of  ten  feet 
or  more.  Assume  the  most  favorable  soil  conditions,  that 


Fig.  93- 

is  to  say,  either  a  good  conductivity  of  the  earth  or  a  stream 
of  water  at  a  short  distance  below  the  surface.  The  lines 
of  force — we  may  call  them  earth  currents  when  within  the 
earth,  since  the  latter  accompanies  the  other — will  be  con- 
centrated around  each  wire  forming  the  network,  and  will 
occasion  losses,  if  the  network  be  buried  or  placed  on 

185 


304]       ELEMENTS   OF  RADIOTELEGRAPHY. 

the  surface.  The  situation  is  diagrammed  in  Fig.  93,  If 
a  counterpoise  be  elevated  above  the  surface  of  the  earth, 
the  condition  shown  in  Fig.  94  will  obtain.  It  will  be  ob- 
served that  in  this  case  the  lines  of  force  all  enter  the  earth 
in  straight  vertical  lines  with  no  crowding  or  concentra- 
tion. Such  a  state  of  affairs  is  exceedingly  advantageous, 
and  is  similar  to  replacing  the  network  with  a  solid  sheet 
of  metal,  everywhere  in  contact  with  the  earth,  of  the  same 
external  dimensions,  and  at  far  less  expense.  Accordingly, 
we  find  that  the  counterpoise  has  been  successfully  em- 
ployed in  high  power  stations  both  in  this  country  and 
abroad.  When  a  counterpoise  is  used,  it  forms  a  large 
capacity  with  the  surface  of  the  earth,  if  it  be  a  good  con- 
ductor, or  with  the  subterranean  water — if  that  be  pres- 
ent near  the  surface.  When  two  condensers  of  greatly 


vr> 


i  ! 


Fig.  94. 

unequal  size  are  placed  in  series,  the  resultant  capacity 
(computed  by  equation  12)  is  approximately  that  of  the 
smaller.  Accordingly,  the  interposition  of  the  capacity  of 
the  counterpoise  in  the  antenna  circuit  does  not  serve  to 
cut  down  the  antenna  capacity  and  wave  length  on  account 
of  the  great  capacity  of  the  counterpoise. 

304.  The  most  satisfactory  "grounds"  are  obtained  on 
ship  installations,  where  direct  connection  can  be  made, 
thru  the  hull  of  the  ship,  with  sea  water — a  perfect  con- 
ductor. 

186 


ANTENNA   RESISTANCE.  [305 

XLV. 

ANTENNA  RESISTANCE. 

305.  The  resistance  of  the  antenna  consists  of  two  parts, 
the  ohmic  or  Joulean  resistance,  and  the  radiation  resist- 
ance. The  former  term  applies  to  that  form  of  resistance 
with  which  we  have  already  become  familiar,  that  is  to  say, 
that  property  of  a  conductor  by  virtue  of  which  heat  is 
formed  in  the  overcoming  of  it  by  the  electric  current. 
(See  paragraph  15.)  Of  the  power  which  is  put  into  the 
antenna,  part  is  expended  or  wasted  in  the  form  of  heat ; 
the  remainder  is  available  for  radiation  purposes.  Since 
the  formula  for  power  from  equation  (4)  is 

P  =  PR, 

we  may  define  radiation  resistance  as  that  quantity  which, 
when  multiplied  by  the  square  of  the  antenna  current,  will 
give  the  power  radiated.  It  is  thus  seen  that  the  term  is 
purely  an  arbitrary  unit  with  no  tangible  or  physical  ex- 
istence. Or  we  may  say  that  the  power  radiated  from  an 
antenna  corresponds  to  energy  used  or  expended  in  a  cir- 
cuit containing  resistance — in  this  case,  the  fictitious  radia- 
tion resistance.  While  the  radiation  resistance  may  be 
measured,  it  should  be  borne  in  mind  that  it  does  not 
really  exist,  it  is  merely  a  term  to  account  for  the  energy 
used  in  radiation — following  the  above  formula.  The  total 
power  in  an  antenna  is  thus  given  by  the  formula 

P  =  PRj  +  I-Rry  (56) 

where  P  is  the  power  in  the  antenna,  /  is  the  antenna  cur- 
rent as  indicated  by  the  antenna  ammeter,  Rj  is  the  Joul- 
ean resistance  of  the  various  conductors  of  the  antenna 
circuit  and  of  the  dielectric  and  ground  within  the  static 
field  of  the  antenna,  and  Rr  is  the  radiation  resistance.  For 

187 


306]      ELEMENTS   OF  RADIOTELEGRAPHY. 

most  efficient  radiation,  it  of  course  follows  that,  other 
things  being  equal,  Rj  should  be  kept  as  low  and  Rr  as  high 
as  possible. 

306.  The  formula  for  radiation  resistance  is  given  by  the 
equation 


Rr=  1607T2-, 


(57) 


where  Rr  is  the  radiation  resistance  in  ohms,  h  is  the 
effective  height  of  the  antenna  in  meters  and  X  represents 
its  wave  length  also  measured  in  meters.  Since  the  square 
of  TT  is  practically  10,  it  may  be  rewritten  as 


RT  =  1600 


(58) 


The  effective  height  of  an  antenna  is  of  course  a  constant, 
but  as  the  wave  length  of  the  antenna  increases  as  it  is 
loaded  for  longer  working  tunes,  the  radiation  resistance 


A 
Fig.  95- 

decreases,  since  X  occurs  in  the  denominator.  When  this 
equation  is  plotted,  it  assumes  the  form  shown  in  Fig.  95, 
approaching  infinity  at  its  lower  limit  of  wave  length,  and 
zero  at  the  upper. 

188 


ANTENNA   RESISTANCE. 


[308 


307.  The  subject  of  antenna  resistance  has  been  investi- 
gated by  L.  W.  Austin,  Head  of  the  U.  S.  Naval  Radio  Lab- 
oratory at  the  Bureau  of  Standards,  and  his  publications 
on  the  subject  have  been  supplemented  by  J.  M.  Miller 
of  the  same  bureau.     Austin  found  that  while  the  radiation 
resistance  of  the  antenna  for  increasing  wave  length  de- 
creased according  to  the  formula  of  equation  58,  the  Joul- 
ean  resistance  of  the  antenna  increased,  instead  of  remain- 
ing constant. 

308.  It  has  been  observed  by  many  investigators  that  in 
a  condenser  employing  an  imperfect  dielectric,  that  is  to 
say,  subject  to  internal  losses  as  is  glass  (air  is  the  most 
perfect  dielectric),  the  equivalent  resistance  varies  directly 
as  the  wave  length.    Its  resistance  is  thus  given  by  the 
equation 

Re  =  aX,  (59) 

where  Re  is  the  equivalent  resistance,  u>ohms,  of  a  capacity 
with  imperfect  dielectric,  a  is  a  constant,  and  X  is  the  wave 


A 
Fig.  96. 

length  of  the  alternating  potential  charging  it.  The  curve 
of  such  an  equation,  as  noted  in  paragraph  254,  is  a  straight 
line  as  shown  in  Fig.  96,  for  with  each  increase  in  wave 
length,  there  is  a  corresponding  increase  in  the  equivalent 
resistance  of  the  condenser.  Now,  while  the  dielectric  of 

189 


309]       ELEMENTS   OF  RADIOTELEGRAPHY. 


an  antenna,  at  first  thought,  would  not  be  considered  im- 
perfect—being air,  it  happens  that  within  the  static  field 
surrounding  it  there  are  often  imperfect  dielectrics  present 
— such  as  wooden  masts,  trees,  stays,  buildings,  etc. 
These,  being  within  the  field  of  the  antenna,  constitute 
part  of  its  dielectric,  and  accordingly  give  to  its  Joulean  re- 
sistance the  shape  of  the  curve  in  Fig.  96. 

309.  From  equation  (56),  it  will  be  observed  that  the 
total  resistance  of  the  antenna  is  composed  of  the  total 
Joulean  resistance  plus  the  radiation  resistance,  or 

Ra  =  Rj  +  Rr,  (60) 

where  Ra  is  the  total  antenna  resistance  as  measured,  Rj  is 
the  total  Joulean  resistance  and  Rr  is  the  radiation  resist- 
ance. The  total  Joulean  resistance,  /?,-,  is  equal  to  the 


A 

Fig.  97. 

equivalent  resistance  of  the  imperfect  dielectric  of  the  an- 
tenna, Re,  and  the  fixed  "ohmic"  resistance  of  the  metallic 
conductors  of  the  circuit  (neglecting  the  skin  effect,  which 
may  be  reduced  to  a  minimum  by  the  use  of  the  wire  de- 
scribed in  paragraph  164),  which  we  will  denote  by  R0. 
This  is  then  written  as 


R0  +  Re. 
190 


(61) 


ANTENNA   RESISTANCE.  [310 

If  we  represent  1,600  h-  of  equation  (58)  by  fr,  and  R0  by 
c,  equations  (58,)  (59),  (60)  and  (61)  may  be  combined  as 
follows  : 

Ra  =  a\+      +  c.  (62) 


Since  c,  the  "ohmic"  resistance  of  the  metallic  conductors 
of  the  antenna  circuit,  is  a  constant  and  is  not  combined 
with  a  variable,  the  last  term  of  equation  (62)  may  be 
ignored  in  plotting  its  curve.  The  curve  of  the  second 
member  of  the  equation  has  been  given  in  Fig.  96,  and 
that  of  the  third  in  Fig.  95.  The  two  may  be  combined 
in  one  figure  as  shown  in  the  dotted  lines  in  Fig.  97,  their 
resultant  being  the  heavy  line.  The  shape  of  this  curve 
has  been  experimentally  verified  many  times,  and  several 
examples  have  been  published  by  Austin.  The  more  per- 
fect the  dielectric  of  the  antenna,  that  is  to  say  —  the  less 
the  amount  of  imperfect  dielectric  introduced  into  the  static 
field  of  the  antenna,  the  flatter  will  be  the  straight  line 
curve  Re,  and  the  less  will  be  the  rise  of  the  total  an- 
tenna resistance  curve,  /?a,  after  its  lowest  point  has  been 
reached. 

310.  It  will  be  seen  that  the  value  of  Re  will  vary  from 
day  to  day  for,  as  the  humidity  and  temperature  change, 
the  resistance  of  the  various  dielectrics  of  the  antenna  will 
be  altered  due  to  the  variation  in  moisture.  This  will 
alter  the  resultant  total  resistance  of  the  antenna.  In  a 
spark  transmitter,  the  damping  of  the  antenna  circuit  of 
which  depends  upon  the  antenna  resistance  as  given  by 
equation  (32)  —  in  which  R  is  replaced  by  Ra  of  the  equa- 
tions above,  this  resistance  variation  will  necessitate  a 
change  in  coupling  in  order  to  compensate  for  the  change 
in  the  current  damping. 

191 


311]      ELEMENTS   OF  RADIOTELEGRAPHY. 

311.  In  paragraph  305,  we  observed  that  PRr  of  equation 
(56)  should  be  kept  at  a  maximum  for  maximum  radiation. 
The  correct  wave  length  for  a  maximum  value  of  this 
quantity  may  be  experimentally  determined  as  follows: 
In  measuring  the  resistance  of  the  antenna,  the  total  re- 
sistance, 7?a,  of  Fig.  97,  is  obtained.     When  a  curve  ap- 
proaches the  vertical  and  horizontal  axes  at  infinite  values 
of  each — as  in  the  case  of  the  radiation  resistance,  see  Fig. 
95 — the  curve  is  said  to  be  asymptotic  to  those  lines.     Thus, 
when  a  curve  or  line  is  asymptotic  to  another  line,  it  meets 
it  at  infinity.     To  separate  Ra  into  its  constituents,  we  may 
lay  off  a  straight  line  asymptotic  to  it,  at  its  upper  limit,  and 
passing  thru  the  zero  point.     This  will  be  Re  of  Fig.  97. 
Since  the  radiation  resistance,  Rr,  equals  the  total  resist- 
ance, Ra1  minus   the  equivalent  dielectric   resistance,  Re, 
it  may  be  obtained   by   subtracting  the   ordinates  of  Re 
from   those   of   Ra.    This   gives   us  a   simple  method  of 
obtaining  the  curve  Rr  of  Fig.  95  without  calculation.     It 
will  be  found  in  practice  that  the  antenna  current  of  a 
transmitter  increases,  as  the  antenna  is  loaded  above  its 
fundamental  or  natural  wave  length,  up  to  a  certain  wave 
length,   and   then   decreases.     This   wave   length   is   not 
necessarily  the  optimum  wave  length  for  radiation  pur- 
poses because,  as  we  have  seen,  the  radiation  resistance 
decreases  at  the  longer  wave  lengths.     Consequently,  the 
optimum  wave  length  will  be  that  for  which   there   is  a 
maximum  antenna  current   consistent  with  the  greatest 
radiation    resistance.      (See    paragraph    305.)     In    other 
words,  that  wave  length  should  be  employed  which  will 
give  a  maximum  value  to  PRr  of  equation  (56). 

312.  The  transmitter  should  now  be  adjusted  for  var- 
ious wave  lengths,  ranging  from  the  fundamental  of  the 
antenna — in  steps  of   25  meters — to  a  wave   length  such 
that  the  antenna  current  has  commenced  to  grow  less. 

192 


WAVE  PROPAGATION.  [314 

Readings  of  the  antenna  ammeter  should  be  recorded  and 
squared.  If  the  values  of  I-  for  the  different  wave  lengths 
of  the  antenna  are  multiplied  by  the  values  of  the  radia- 
tion resistance  at  the  same  wave  lengths — as  determined 
by  the  Rr  curve  obtained  according  to  the  procedure  out- 
lined in  the  preceding  paragraph — the  optimum  wave 
length  will  be  that  at  which  I~Rr  is  the  greatest.  This  wave 
length  is  not  necessarily  that  which  will  give  the  greatest 
response  at  a  distant  receiving  station,  for  this  is  dependent 
upon  many  other  factors,  but  the  wave  length  for  the  great- 
est amount  of  radiated  energy  may  be  determined  by  this 
method. 

XLVI. 
WAVE  PROPAGATION. 

313.  In  the  original  experiments  of  Hertz,  an  ungrounded 
oscillator  was  used,  that  is  to  say,  an  open  oscillating  cir- 
cuit similar  to  the  antenna  of  the  Marconi  1896  patent, 
except  that  it  was  not  connected  to  earth  at  one  end.     This 
radiating  circuit  resulted  in  the  emission  of  ungrounded 
waves,  in  other  words,  waves  that  did  not  travel  with  their 
feet  on  the  ground  as  did  the  Marconi  waves.     (See  para- 
graph 69.)     Accordingly  they  were  radiated  in  straight  lines, 
similar  to  light  waves.     They  could  not  have  been  used  for 
transmission  over  great  distances,  even  if  sufficient  power 
had  been  available,  for  there  would  have  been  no  tendency 
of  the  Hertzian  waves  to  follow  the  curvature  of  the  earth. 
Due  to  the  straight  line  nature  of  their  transmission,  when  an 
obstruction  was  encountered  in  their  path,  a  region  of  elec- 
trical shadow  was  left  behind,  similar  to  the  shadow  caused 
by  an  opaque  substance  interposed  in  a  beam  of  sunlight. 

314.  With   the   basic   invention   of    Marconi,  however, 
which  was  the  first  to  employ  a  grounded  radiating  circuit, 

193 


315]       ELEMENTS   OF  RADIOTELEGRAPHY. 

lines  of  force  are  set  up  between  the  antenna  and  the 
earth,  as  we  have  previously  seen.  These  become  de- 
tached from  the  antenna  and  are  converted  into  free  waves, 
and,  being  grounded  at  their  lower  extremities,  are  guided 
by  the  surface  of  the  earth.  (See  Fig.  11.)  The  curva- 
ture of  the  earth  accordingly  offers  no  hindrance  to  the 
propagation  of  radio  waves  over  tremendous  distances, 
and  when  an  obstruction  is  reached  —  such  as  a  mountain, 
the  waves  pass  up  one  side  and  down  on  the  other.  The 
effect  of  the  electrical  shadow  accordingly  is  minimized. 

315.  The  radiation  of  waves  from  an  antenna  is  shown  in 
Fig.  98.  It  will  be  noted  that  this  is  similar  to  Fig.  11. 
The  distance  between  the  beginnings  of  any  two  waves  in 
which  the  lines  of  force  have  the  same  direction  measures 
the  wave  length,  represented  by  X  in  the  figure.  It  will 


\ 


—  A--  —  H 

Fig.  98. 


be  seen  in  the  figure  that  the  wave  is  not  detached  from 
the  antenna  until  it  is  one  quarter  of  a  wave  length  away. 
These  travelling  lines  of  force  in  the  atmosphere,  since 
they  are  grounded  at  their  lower  extremities,  are  accom- 
panied by  earth  currents.  We  may  think  of  the  phenom- 
enon as  consisting  of  projected  earth  currents  accompanied 

194 


WAVE  PROPAGATION.  [316 

by  electrostatic  strains  in  the  ether,  or  of  lines  of  electro- 
static force  accompanied  by  currents  in  the  earth.  The 
net  result  is  the  same.  The  two  forms  of  radiation  may 
not  be  separated.  Accompanying  the  electrostatic  radi- 
ation, there  is,  of  course,  electromagnetic  distortion  of 
the  ether  as  well,  or  lines  of  magnetic  flux.  These  are  at 
right  angles  to  the  electrostatic  field,  so  that  the  plane  of 
their  projection,  instead  of  being  perpendicular  to  the 
earth's  surface — as  is  that  of  the  electrical  waves,  is  par- 
allel to  it. 

316.  In  order  that  the  resistance  offered  to  the  ground 
currents  may  be  as  low  as  possible,  it  is  desirable  that  the 
conductivity  of  the  guiding  surface  of  the  waves  be  ex- 
tremely high.  Thus,  transmission  over  sea  water  may  be 
more  easily  effected  than  over  land,  because  salt  water  is 
practically  a  perfect  conductor.  In  such  a  case,  the  plane 
of  the  radiated  wave,  near  the  surface,  is  perpendicular  to 
that  of  the  guiding  surface,  as  shown  in  Fig.  99  a.  (This 
figure  is  reproduced  from  Zen- 
neck's  discussion  of  the  subject.) 
In  the  case  of  transmission 
overland,  the  same  situations 
would  obtain  if  the  conductivity 
of  the  soil  were  equal  to  that  of  Sea  Wafer 
sea  water.  Such  is  rarely,  if 
ever,  the  case,  and  instead  we  Fig.  99. 

find  the  lower  part  of  the  wave, 

due  to  the  increased  resistance  of  the  earth,  lagging  behind 
the  upper  portion,  as  in  Fig.  99  b.  The  fact  that  the  wave 
is  now  inclined  and  is  not  vertical,  means  that  it  may  be 
resolved  into  two  waves,  one  producing  a  vertical  static  field 
and  the  other  a  horizontal  one.  (In  paragraph  37,  we  ob- 
served that  two  forces  at  right  angles  may  be  combined  into 

195 


317]       ELEMENTS   OF  RADIOTELEGRAPHY. 

a  resultant,  obtained  by  taking  their  diagonal.  Similarly,  a 
force  exerted  in  an  oblique  direction — as  in  this  case- 
may  be  considered  as  the  resultant  of  two  forces  acting  at 
right  angles  to  each  other.)  The  horizontal  field  is  parallel 
to  the  earth's  surface  and  causes  energy  from  the  wave  to 
be  absorbed  therein,  decreasing  the  distance  of  transmis- 
sion. 

317.  As  the  resistance  of  the  soil  is  increased,  a  complete 
change  in  the  situation  takes  place.     Instead  of  causing  a 
greater  lag  in  the  lower  portion  of  the  wave,  when  the  sur- 
face over  which  the  wave  is  transmitted  is  a  perfect  insu- 
lator, no  absorption  of  ground  currents  can  take  place  and 
there  is  no  lagging  effect.     While  the  plane  of  the  electro- 
static field  is  once  more  perpendicular  to  the  earth's  sur- 
face, there  is  no  guiding  influence  exerted  upon  it,  and  as 
a  consequence,  the  waves  becoming  ungrounded,  revert  to 
Hertzian  waves  and  are  projected  in  straight  lines,  no 
longer  following  the  curvature  of  the  earth. 

318.  It  is  apparent,  then,  that  for  maximum  distance  of 
transmission,  conductivity  of  the  surface  over  which  the 
waves  are  to  pass  plays  an  important  part. 

319.  To  absorb  the  most  energy  from  a  given  wave  at 
the  receiver,  it  is  necessary  to  intercept  as  much  of  it  as  pos- 
sible in  order  that  the  greatest  difference  of  potential  from 
the  grounded  portion  to  its  upper  limit  may  be  impressed 
upon  the  receiving  antenna.     Hence,  the  higher  the  re- 
ceiving antenna,  the  greater  the  distance  over  which  signals 
will  be  received.     Kites  have  been  used,  flown  at  heights 
of   many   hundred   feet,   with   great   success.     With   the 
ground  antenna  described  in  the  early  part  of  this  chapter, 
on  the  other  hand,  use  is  made  not  of  the  vertical  static 
field  of  the  wave  but  of  the  horizontal  component  (see 
paragraph  316)  and  of  the  wave's  accompanying  earth  cur- 

196 


WAVE  PROPAGATION.  [321 

rents.  Atmospheric  discharges,  if  they  do  not  consist  of 
a  direct  discharge  to  earth,  the  so-called  "thunderbolt," 
give  rise  to  Hertzian  waves  at  their  origin.  As  soon  as 
they  reach  the  surface  of  the  earth,  however,  they  become 
the  familiar  grounded  Marconi  waves,  and  as  such  may 
affect  the  ground  antenna.  Only  those  atmospheric  waves 
which  have  originated  at  points  sufficiently  far  away  to  be- 
come grounded  can  thus  affect  the  ground  antenna,  and 
their  intensity  accordingly  is  not  very  great. 

320.  It  has  long  been  observed  that  for  a  given  power, 
greater  distances  of  transmission  may  be  effected  at  night 
than  in  the  daytime,  and  over  water  than  over  land.     Also 
at  dusk  and  at  dawn,  marked  diminution  of  signals  occur. 
These  phenomena  have  been  variously  but  not  conclusively 
explained.  While  we  might  expect  more  distant  transmission 
during  the  hours  of  darkness  over  land  than  in  the  daylight 
hours,  on  account  of  the  increased  conductivity  of  the  earth 
due  to  the  moisture  collected  on  its  surface,  this  does  not 
account  for  a  similar  increase  in  the  strength  of  signals 
for  night  transmission  over  water,  where  the  conductivity 
of  the  guiding  surface  is  constant  for  day  and  night  trans- 
mission.    It  is  probable  that  increase  in  the  strength  of 
signals  at  night  is  due  to  amplification  of  both  factors  of 
the  propagation,  i.e.,  increase  in  the  earth  currents  and  in- 
crease in  the  space  wave.     The  latter  may  be  due  to  a 
variety  of  factors,  which  we  will  analyze. 

321.  We  have  observed  sunlight,  with  its  ultra-violet 
rays,  to  be  a  prolific  source  of  ionization.     Accordingly,  in 
daylight,  the  increased  conductivity  of  the  atmosphere  due 
to  the  sun's  ionization  may  be  responsible  for  considerable 
absorption  of  the  wave.     This  explanation  is  not  wholly 
satisfactory,  however,  for  the  conductivity  of  the  air — even 
with  the  ionization  of  the  sun's  rays — is  not  great  enough 

197 


322]       ELEMENTS   OF  RADIOTELEGRAPHY. 

to  produce  so  marked  a  decrease  of  signals  as  is  observed. 
The  situation  is  further  clouded  by  the  fact  that  when  using 
undamped  waves  on  certain  wave  lengths  between  two 
stations,  better  results  are  actually  obtained  in  daylight. 
That  the  phenomenon  of  reflection  appears  to  play  no  small 
part  in  the  matter  appears  likely. 

322.  As  we  ascend  higher  and  higher  into  the  air,  a  dim- 
inution of  pressure  is  quite  apparent.  This  decrease  in 
the  atmospheric  pressure  brings  with  it  lowered  resistance. 
(See  paragraph  105  on  the  conductivity  of  compressed  air.) 
As  greater  and  greater  heights  are  attained,  a  state  of  par- 
tial vacua  is  reached,  which,  coupled  with  the  increased 
intensity  of  the  ultra-violet  rays  from  the  sun  at  this  ele- 
vation, is  responsible  for  a  condition  of  excellent  conduc- 
tivity. We  may  assume,  then,  for  the  purposes  of  this  dis- 
cussion, that  at  a  great  distance  above  the  surface  of  the 
earth  is  a  conducting  layer.  If  a  radio  wave  should  strike, 
in  its  flight,  a  metallic  shield  or  screen  of  tremendous  area, 
it  would  be  reflected  from  it,  just  as  light  is  reflected.  This 
conducting  layer  above  the  earth  similarly  serves  to  reflect 
radio  waves  back  to  the  earth.  A  reflected  wave  from  a 
station  cannot  arrive  at  the  receiving  station  as  soon  as  the 

••Direct  Wave 

-  Reflected 


Fig.  100. 

direct  wave  which  followed  its  normal  path  along  the  surface 
of  the  earth  for  it  has  had  a  greater  distance  to  travel— 
from  the  transmitter  up  to  the  reflecting  layer  and  thence 
back  to  the  earth  again,  If  the  reflected  wave  arrives  at 
the  receiver  one  half  period,  or  a  multiple  thereof  (see  para- 

198 


WAVE  PROPAGATION.  [323 

graph  64)  behind  the  original  or  direct  wave,  the  condition 
shown  in  Fig.  100  will  obtain.  That  is  to  say,  the  two  waves 
will  be  exactly  opposite  in  phase— one  will  be  at  a  maxi- 
mum in  one  direction  when  the  other  will  be  at  a  maximum 
in  the  other.  This  phenomenon  is  termed  wave  interfer- 
ence. In  the  figure,  the  direct  wave  is  shown  in  the  heavy 
line  and  the  reflected  one  in  the  dotted  line.  It  will 
be  observed  that  these  two  waves  will  cancel  each  other 
and  no  signals  will  be  heard.  Such  a  state  of  affairs  can- 
not be  realized,  however,  for  the  reflected  wave  has  never 
as  much  energy  as  the  direct  wave,  since  it  has  had  a  greater 
distance  to  travel.  So  that  even  with  conditions  most  un- 
favorable, i.e.,  the  two  waves  exactly  opposite  in  phase,  sig- 
nals will  still  be  heard.  On  the  other  hand,  should  the  re- 
flected wave  arrive  in  phase  with  the  direct  wave,  the  time 
of  lag  being  an  exact  or  integral  multiple  of  the  time  period, 
a  reinforcement  of  the  direct  wave  by  the  reflected  one  will 
take  place  and  an  increase  in  signals  will  occur.  By  changing 
the  wave  length  of  the  transmitter  slightly,  interference  be- 
tween the  two  waves  at  the  receiver  may  be  changed  to  rein- 
forcement. This  is  very  often  done  in  long  distance  trans- 
mission with  the  Poulsen  arc  where  the  use  of  a  single  circuit 
transmitter  makes  a  minute  variation  in  the  emitted  wave 
a  simple  matter.  As  a  matter  of  fact,  it  is  quite  essential 
that  means  be  provided  for  making  such  a  slight  change 
in  wave  length  if  it  is  desired  to  transmit  over  long 
distances  thru  the  dusk  and  dawn  periods,  at  which 
times  the  reflection  phenomenon  appears  to  be  the  most 
pronounced. 

323.  The  subject,  as  may  be  seen  from  even  the  ele- 
mentary considerations  presented  above,  is  quite  complex, 
and  a  further  discussion  of  refraction  and  other  factors  in- 
volved will  not  be  presented  in  this  text. 
16  199 


324]       ELEMENTS   OF  RADIOTELEGRAPHY. 

XLVII. 
AERIAL   COMMUNICATION. 

324.  Under  this  somewhat  ambiguous  heading  will  be 
presented  a  discussion  of  the  "ways  and  means"  of  radio 
communication  with  air  craft.     Since  it  is  not  possible  to 
obtain  a  connection  to  earth,  various  expedients  must  be 
used  to  provide  a  counter  capacity  for  the  antenna. 

325.  In  seaplanes  or  flying  boats,  a  long  wire  for  the  an- 
tenna is  trailed  from  the  plane,  dropped  thru  an  insulated 
duct  in  the  fusilage.     A  leaden  weight  is  fastened  at  one 
end,  permitting  it  to  be  quickly  lowered,  and  it  is  reeled  in 
manually  by  the  military  observer  or  by  the  pilot,  if  a  single 
machine.    "Ground"  connection  is  made  to  the  engine,  the 
wire  stays  between  the  wings,  and  such  other  metal  work  as 
will  not  subject  the  occupants  to  shock.     We  have  observed 
in  the  earlier  parts  of  this  chapter  that  no  potential  exists  at 
the  lower  terminus  of  the  antenna  circuit,  since  the  capacity 
of  the  ground  is  so  great  that  for  the  amount  of  charge  im- 
parted to  it  by  the  average  antenna  current  only  very  minute 
fluctuations  in  the  potential  at  this  point  in  the  circuit  can 
occur.     The  capacity  of  the  "ground"  connection  for  air 
craft,  on  the  other  hand,  is  quite  small  and  by  no  means 
comparable  with  that  of  the  earth.     Accordingly,  the  same 
conditions   obtain   with   this   type   of   antenna   circuit   as 
with  the  original  Hertz  oscillator,  as  shown  in  the  figure 
at  the  top  of  Fig.  11.     (See  paragraph  313.)     That  is  to 
say,  both  members  of  the  antenna  circuit,  the  aerial  and 
the  "ground,"  are  charged  to  high  potentials.     For  this 
reason,  it  is  essential  in  radio  installations  on  dirigibles 
that  no  sparking  occur  from  the  "ground"  to  stays  or  other 

200 


AERIAL   COMMUNICATION.  [328 

metal  work,  as  the  danger  of  ignition  of  the  gas  in  the  bag 
would  be  very  great.* 

326.  Since  the  antenna  circuit  is  similar  to  that  of  the 
original  Hertz  transmitter,  it  follows  that  the  waves  radi- 
ated from  air  craft  travel  in  straight  lines  and,  in  general, 
behave  similar  to  those  of  light.     (See  paragraph  313.) 
However,  if  the  seaplane  should  be  a  considerable  dist- 
ance from  the  receiver,  the  Hertzian  waves  which  it  would 
radiate  would  probably  strike  the  earth  before  reaching  the 
receiver,  in  which  case  they  would  become  similar  to  the 
familiar  grounded  waves  of  Marconi.     The  Hertzian  nature 
of  the  radiation  from  a  seaplane |  is  noticeable  when  the 
machine  is   turning  for,  under  certain  conditions,  signals 
are  greatly  weakened. 

327.  The  securing  of  an  adequate  source  of  power  on 
air  craft  is  a  vexing  problem.     The  lifting  power  of  a  sea- 
plane carrying  an  observer,  a  machine  gun  and  several 
bombs  is  greatly  reduced.     Further,  it  is   not   advisable 
to  couple   a  generator  to  the   motor.    A  type  of  light, 
windage   generator    has    been   devised,   consisting   of    a 
small  generator  on  the  shaft  of  which  is  mounted  a  pro- 
peller.    This  is  revolved  by  the  wind  generated  in  flight. 
The  correct  location  of  this  generator  is  paramount,  for 
the  increased  head  resistance  to  the  plane  caused  by  the 
generation  of  as  low  as  one  quarter  kilowatt  is  sufficient  to 
tilt  the  plane  usually  upwards,  if  the  dynamo  be  not  cor- 
rectly placed.     The  lifting  power  of  a  dirigible  is  so  great 
that  the  installation  of  a  small  power  plant  thereon  is 
more  easily  effected. 

328.  Receiving  operations  on  a  dirigible  may  be  carried 

*  The  growing  use  of  some  of  the  rarer  gases,  such  as  helium,  which 
are  non-inflammable,  will  obviate  this  danger. 

f  The  term  "  seaplane  "  is  intended  to  include  flying  boats,  as  well, 

201 


328]       ELEMENTS   OF  RADIOTELEGRAPHY. 

on  without  much  difficulty,  for  the  operating  cabin  can  be 
located  at  a  considerable  distance  from  the  motors  and 
propellers.  On  a  plane,  on  the  other  hand,  space  is  ex- 
tremely limited,  and  the  operator  is  subjected  to  noise 
from  the  unmuffled  motor.  With  pneumatic  ear  devices 
and  sufficient  amplifying  apparatus,  however,  signals  may 
be  sent  to  a  sea  plane  if  the  distance  of  transmission  be 
not  too  great.  Transmission  from  the  plane  is  easily  ef- 
fected; distances  as  great  as  125  miles  having  been  covered 
on  the  Pacific  Coast  in  1916.  A  distance  of  1600  miles  was 
covered  by  radio  from  a  Zeppelin  type  dirigible  in  1919. 


202 


CHAPTER  TEN. 

XLVIII. 
PIONEER  RECEIVERS. 

329.  In  the  consideration  of  receivers,  with  which  the 
rest  of  this  text  will  be  concerned,  it  is  helpful  to  remember 
that  the  development  of  the  various  forms  of  receivers  was 
coincident   with   that  of   transmitters.     Accordingly,   fre- 
quent comparison  should  be  made  by  the  reader  between 
the  forms  of  receivers  described  below  and  the  transmitters 
with  which  they  were  patented,  developed,  and  used. 

330.  Thus,  with  the  Marconi  plain  aerial,  1896  transmit- 
ter, we  find  a  receiver  of  very  similar   construction.     It 


Fig.  101.     Marconi  1896  Receiver. 

is  shown  in  Fig.  101,  in  which  C  represents  a  coherer;  Z,, 
choke  coils;  Bl  and  B2j  batteries;  R,  a  relay,  and  S,  a  Morse 
telegraph  sounder.  The  operation  was  as  follows:  The 
coherer  consisted  of  a  glass  tube  containing  metal  filings 

203 


331]      ELEMENTS   OF  RADIOTELEGRAPHY. 

loosely  packed  between  the  electrodes  A  and  B.  Ord- 
inarily, the  coherer  had  a  very  high  resistance  due  to  the 
fact  that  the  filings  lay  loosely  in  the  tube  and  did  not 
make  perfect  contact  with  each  other.  When  a  current  of 
radio  frequency  was  passed  thru  them,  however,  their 
resistance  was  greatly  lowered.  A  tapper,  consisting  of 
the  clapper  of  an  electric  bell — not  shown — was  arranged 
to  strike  the  tube  lightly,  so  that  the  filings  would  fall 
apart  when  the  radio  current  had  ceased  to  pass  thru  them, 
thus  causing  them  to  regain  their  original  high  resistance. 
The  battery  BI  served  to  furnish  direct  current  thru  the 
coherer.  Ordinarily,  the  resistance  of  the  coherer  was 
so  high  that  the  battery  was  not  able  to  trip  the  relay  R. 
When  the  radio  current  from  the  incoming  wave  passed 
thru  the  coherer,  its  resistance  was  lowered  sufficiently  to 
permit  the  current  to  operate  the  relay.  The  relay  con- 
trolled, in  turn,  a  Morse  sounder  or  tape  recorder,  on  which 
the  dots  and  dashes  of  the  code  were  read.  The  coherer 
thus  acted  as  a  valve  or  trigger,  releasing,  under  the  in- 
fluence of  the  incoming  radio  frequency  current,  the  local 
current  to  actuate  the  relay.  (The  choke  coils  L  served  to 
keep  the  radio  frequency  current  out  of  the  local  circuits, 
altho  they  did  not  hinder  the  flow  of  direct  current  into 
the  coherer.  See  paragraph  260.) 

331.  This  receiver  was  similar  to  the  1896  transmitter 
of  Marconi  in  the  following  particulars:  there  were  no 
means  provided  for  tuning  the  antenna  circuit;  and,  due 
to  the  inclusion  in  the  antenna  circuit  of  the  very  high  re- 
sistance of  the  coherer,  high  damping  of  the  received  an- 
tenna current,  and  hence  broad  tuning,  resulted.  As  with 
the  transmitter,  it  had  the  great  advantage,  however,  of 
being  a  single  circuit  receiver^  that  is  to  say — there  was 
only  one  oscillating  circuit.  If  there  were  two  oscillating 

204 


PIONEER  RECEIVERS.  [333 

circuits  coupled  together,  as  we  shall  find  in  the  Marconi 
1900  receiver,  the  receiving  system  would  be  responsive  to 
waves  of  double  frequency — it  would  in  itself  tend  to  oscil- 
late at  two  frequencies — and  reduced  efficiency  would  re- 
sult. 

332.  Lodge,  in  his  1898  patent,  improved  upon  the  Mar- 
coni 1896  receiver  just  as  he  did  with  the  transmitter.     In 
the  antenna  circuit,  he  inserted  inductance  coils  to  reduce 
the  damping  of  the  received  antenna  current,  and  he  re- 
moved the  coherer  from  that  circuit  to  reduce  the  resistance. 
(See  paragraph  82.)     This  permitted  him  to  tune  the  re- 
ceiver, or  resonator ,  as  he  termed  it,  so  that  the  inductive 
and   capacity  reactances   of  the   antenna  would  balance 
each  other  for  the  frequency  of  the  received  currents,  and 
the  receiver  and  transmitter  would  thus  be  placed  in  res- 
onance.    In  removing  the  coherer  from  the  antenna  cir- 
cuit  to   another   circuit   inductively   coupled   thereto,   he 
greatly  reduced  the   antenna  resistance   and  hence   the 
damping  of  the  received  current.     As  a  result,  he  not  only 
had  tuning  between  his  transmitter  and  receiver,  but  he 
had  sharp  tuning,  the  essential  factors  for  successful  radio 
communication. 

333.  A  diagram  of  the  circuits  which  he  employed  is 
shown  in  Fig.  102.     LI  is  the  antenna  inductance  which 
played  the  triple  role  of  tuning  the  antenna  to  resonance 
with  the  incoming  oscillations,  reducing  the  decrement  of 
the  antenna,  and  serving  as  a  means  of  inducing  energy 
into  the  coherer  circuit.     L2  serves,  by  means  of  the  mutual 
induction  with  JLi,  to  receive  the  energy  from  the  antenna 
circuit.     C  represents  the   coherer,  a  type   invented  by 
Lodge  which  was  used  in  conjunction  with  a  pair  of  tele- 
phone receivers  T  in  which  the  signals  were  made  audible, 
as  is  the  practice  today.     B  furnished  the  current,  which 

205 


334]       ELEMENTS   OF  RADIOTELEGRAPHY. 

was  triggered  thru  the  coherer   by  the  incoming  energy. 
K  is  a  condenser  of  large  capacity. 


Fig.  102.     Lodge  1898  Receiver  (Untuned  Secondary). 

334.  The  operation  of  the  receiver  is  as  follows:  The 
antenna  circuit  is  tuned  by  the  variable  inductance  LI  so 
as  to  give  the  greatest  value  to  the  radio  frequency  currents 
flowing  therein  resulting  from  the  potentials  impressed  on 
the  circuit  by  the  passing  waves.  These  induce  alternating 
potentials  across  the  terminals  of  the  coil  £2.  With  a  po- 
tential of  radio  frequency  resident  upon  the  terminals  of  a 
circuit,  the  nature  of  the  ensuing  current  depends  upon  the 
inductance,  capacity  and  resistance  of  this  circuit,  as  we  have 
previously  observed.  On  account  of  the  high  resistance  of 
the  coherer  and  the  large  capacity  of  the  condenser  X",  the 
detector  circuit  is  thus  not  an  oscillating  circuit,  but  an 
aperiodic  one.  (See  paragraph  83.)  The  current  flows  in 
impulses,  the  proportion  of  resistance  and  capacity  with  refer- 
ence to  the  inductance  being  too  great  to  permit  of  oscilla- 
tions. (See  paragraph  62.)  Since  the  detector  circuit  is  an 
impulse  circuit,  like  the  impulse  circuit  of  the  Lodge  trans- 

206 


PIONEER  RECEIVERS. 


[335 


mitter,  there  is  no  necessity  for  tuning  it  to  resonance  with 
the  antenna  circuit.  This  is  a  great  advantage  in  receiv- 
ing, for  it  requires  only  the  tuning  of  the  antenna  to  secure 
the  maximum  response  in  the  telephone  receivers.  This 
single  (oscillating)  circuit  type  of  receiver  is  now  hi  use 
with  those  forms  of  spark  transmitters  which  radiate  a 
single  wave  of  feeble  damping,  in  particular — the  Kilbourne 
and  Clark,  Haller  Cunningham,  Telefunken  and  Multitone 
systems  described  in  Chapter  Six.  The  modern  form  of 
this  receiver  is  termed  the  untuned  secondary,  for  no  means 
are  provided,  or  needed,  for  tuning  it  to  resonance  with  the 
antenna  circuit. 

335.  In  1900  Marconi  brought  out  the  receiver  shown  in 
Fig.  103  as  a  companion  to  his  coupled  tuned  circuit  trans- 
mitter. The  antenna  consists  of  the  loading  coil  L,  the 


Fig.  103.     Marconi  1900  Receiver. 
207 


336]       ELEMENTS   OF  RADIOTELEGRAPHY. 

primary  LI  of  the  oscillation  transformer  or  coupler,  which  is 
shunted  by  the  variable  condenser  Ci,  for  increasing  the 
wave  length  of  the  antenna  or  primary  circuit.  £2  repre- 
sents the  secondary  of  the  antenna  coupler  which  includes 
in  its  circuit  the  loading  inductances  g  for  increasing  the 
wave  length  of  the  secondary  circuit.  The  variable  con- 
denser C2  serves  the  same  purpose  and  provides  a  low 
reactance  path  for  the  radio  frequency  current  thru  the  sec- 
ondary circuit,  £2,  <7,  C2,  g.  The  condenser  K  is  a  stopping 
condenser,  serving  to  prevent  the  local  battery  current  from 
flowing  thru  the  secondary,  L2.  On  account  of  its  large 
capacity,  this  condenser  offers  little  impedance  to  the  radio 
frequency  current,  but,  as  we  have  previously  observed,  it 
is  an  open  circuit  to  direct  current.  The  coherer  and 
telephone  receivers  of  the  Lodge  patent  are  shown. 

336.  In  the  complete  Lodge  system,  that  is  to  say — in 
both  transmitter  and  receiver,  there  are  two  tuned  cir- 
cuits— the  antenna  circuits  at  either  end.     With  the  Mar- 
coni system,  there  are  four  tuned  circuits — the  primary  and 
secondary  circuits  at  both  transmitter  and  receiver.     In 
the  latter  system,  it  is  necessary  that  all  four  circuits  be 
tuned  to  the  same  wave  length  before  signals  may  be  ex- 
changed.    The  Marconi  type  of  receiver  is  a  two  circuit 
receiver  in  that  it  has  two  oscillating  circuits.     The  detector 
circuit,  comprising  the  battery,  coherer  and  telephone  re- 
ceivers, forms  a  third  circuit 

337.  Such  a  receiver,  like  its   companion  transmitter, 
has  two  oscillation  frequencies,  due  to  the  coupling  to- 
gether of  two  oscillating  circuits.     (See  Section  XI  on  coup- 
led circuits.)     That  is  to  say,  for  medium  and  close  coup- 
ling, it  will  be  responsive  to  waves  of  two  frequencies.     In 
a  modern  transmitter,  a  single  wave  is  radiated — hence, 
for  efficient  reception,  it  is  necessary  that  the  receiver  os- 
cillate at  but  one  frequency.     This  may  be  approximated 

208 


DETECTORS.  [339 

by  loosening  the  coupling  of  the  primary  and  secondary 
coils  of  the  receiving  transformer  so  as  to  bring  the  two 
oscillation  periods  of  the  receiver  together.  However, 
since  every  change  in  coupling  entails  a  change  in  the 
mutual  induction  between  the  two  circuits  and  variations 
in  the  self-induction  of  each,  it  is  necessary  that  the  pri- 
mary and  secondary  circuits  be  carefully  retuned  to  res- 
onance for  every  coupling  change.  This  makes  tuning  of  a 
two  circuit  receiver  a  very  critical  process,  the  operator 
being  required  to  make  several  adjustments  of  three  vari- 
able factors— primary  wave  length,  secondary  wave  length, 
and  coupling — before  signals  can  be  received  at  their  max- 
imum intensity.  This  procedure  is  in  marked  contrast  to 
that  of  the  untuned  secondary  receiver.  With  the  latter, 
there  is  no  deleterious  effect  when  the  two  windings  of  the 
antenna  coupler  are  closely  coupled,  since  there  is  but  one 
oscillating  circuit.  Accordingly,  the  coupling  may  be  left 
quite  close  for  a  maximum  transfer  of  energy,  and  there  is 
but  a  single  circuit — the  antenna— to  be  adjusted. 

XLIX. 

DETECTORS. 

338.  The  modern  receivers  differ  from  the  Lodge  1898 
and  the  Marconi  1900  receivers  chiefly  in  the  form  of  de- 
tector used,  the  coherer  having  been  superseded  for  many 
years  by  various  other  devices.     Some  of  them  are  of 
interest  solely  from  their  historical  standpoint — others  are 
in  use  today.     They  will  be  described  below. 

339.  Electrolytic. — This  detector  was  invented  by  Fes- 
senden  and  consists  of  a  small  glass  cup  containing  an 
electrolyte — a  dilute  solution  of  sulphuric  acid  (H2SO4)  or 
of  nitric  acid   (HNO3).'    Contact  with  the  electrolyte  is 
made  thru  a  platinum  plate  at  the  bottom  of  the  cup.     Into 

209 


339]       ELEMENTS  OF  RADIOTELEGRAPHY. 


the  acid  is  dipped  a  platinum  wire,  as  shown  in  Fig. 
104.  When  potential  is  applied  across  the  electrodes  A 
and  B  of  the  solution,  the  dissociation  that  ensues  (see 
Section  XIII)  causes  bubbles,  or  a  film,  of  oxygen  to  col- 
lect around  the  positive  electrode  A,  and  a  film  of  hydrogen 
around  the  cathode  B.  (In  the  chemical  symbols  above,  H 
represents  hydrogen;  S,  sulphur;  O,  oxygen;  and  N  stands 
for  nitrogen.  The  hydrogen  ion,  H,  is  always  positive 
and  the  SO4  and  NO3  ions  are  negative.)  These  gaseous 
films  form  an  insulation  to  the  passage  of  current  and  the 
electrolyte  is  said  to  be  polarized.  When  the  potential 
across  the  solution  is  increased,  however,  a  value  will 

be  found    that  overcomes    the 

Jl          polarization.    While  polarization 
exists,  the  cell  obviously  serves 
An          + as  a  condenser,  the   electrodes 
-"T        A  and  B  being  the  plates.     Ac- 
cordingly, if  a  small  alternating 
potential  be  impressed  upon  the 
solution,  the   capacity  effect  of 
the  plates  will  permit  a  current 
to  pass  in  either  direction.     If 
the  size  of  the  anode  be  reduced 

to  a  very  fine  wire,  the  capacity  of  the  cell  will  be  prac- 
tically nil.  If  a  continuous  potential  be  used  to  bring  the 
polarization  of  the  cell  to  a  point  just  below  the  break-down 
voltage,  and  an  alternating  potential  be  superimposed, 
alternating  current  will  be  passed  from  A  to  B  with  the 
battery  current,  but  there  will  be  no  capacitive  effect  to 
permit  it  to  flow  in  the  opposite  direction.  Instead,  when 
the  alternating  potential  is  reversed,  it  becomes  counter  to 
that  of  the  battery  and  the  resultant  potential  is  still  further 
reduced  from  the  breakdown  voltage,  allowing  no  current 
to  pass.  The  polarized  cell  thus  serves  as  a  rectifier  of 

210 


Fig.  104.     Electrolytic 
Detector. 


DETECTORS.  [341 

small  alternating  currents  of  low  potential.  In  practice,  the 
diameter  of  the  anode  wire  is  reduced  to  0.0001  of  an  inch 
to  quite  thoroughly  eliminate  all  capacity  effects.  Such  wire, 
of  course,  is  very  difficult  to  handle  on  account  of  its  small 
diameter.  Commercially,  the  platinum  wire  is  plated  with 
silver.  The  silver  wire  with  its  platinum  core  is  then  drawn 
thru  successive  dies  until  the  proper  size  is  obtained.  The 
wire  is  then  dipped  into  the  solution  and  the  silver  is  eaten 
away  by  the  nitric  acid,  leaving  the  fine  platinum  core  ex- 
posed. Such  wire  is  called  Wollaston  wire. 

340.  While  this  detector  was  the  most  sensitive  one  in 
use  at  the  time  of  its  greatest  popularity — about  1906  to 
1910 — it  had  the  disadvantages  of  being  unsuited  for  work 
aboard  ship  on  account  of  jarring  imparted  to  the  cup  and 
the  acid  which  it  contained.     In  addition,  it  had  to  be 
frequently  adjusted  to  keep  just  the  point  of  the  wire  in 
contact  with  the  electrolyte — in  order  to  compensate  for 
its  evaporation,  and  it  was  easily  burned  out  by  signals 
from  near-by  stations.     An  attempt  was  made  by  the  Mas- 
sie  Wireless  Telegraph  Company — an  American  concern 
operating  on  the  Marconi  patents — to  obviate  these  diffi- 
culties by  sealing  the  Wollaston  wire  in  a  glass  tube  which 
could  be  immersed  to  any  depth  in  the  solution.     This 
procedure,  however,  resulted  in  a  diminution  of  the  sensi- 
tivity of  the  instrument. 

341.  A  diagram  of  the  operating  connection  of  this  de- 
tector is  shown  in  Fig.  105,  where  L  represents  the  re- 
ceiving transformer,  VC  is  a  variable  condenser  for  tuning 
purposes,  SC  is  a  fixed  stopping  condenser  for  preventing 
direct  current  from  the  battery  E  from  passing  thru  the 
secondary   winding   of  the   receiving  transformer,    D    is 
the  electrolytic  detector  and  T  is  the  pair  of  telephone 
receivers.    Across  the  battery  is  shunted  a  variable  resist- 

211 


342]       ELEMENTS   OF  RADIOTELEGRAPHY. 

ance,  termed  a  potentiometer,  for  varying  the  potential 
across  the  detector  so  as  to  keep  it  at  a  state  of  polariza- 
tion just  below  the  break-down  voltage.  It  will  be  seen 
that  when  the  slider  of  the  potentiometer  is  at  the  point  A, 
the  full  potential  of  the  battery  will  be  impressed  upon 
the  detector,  when  at  the  point  B,  there  will  be  no  poten- 
tial, and  when  at  the  half-way  position  C,  the  voltage  will 


Fig.  105.     Receiving  Circuit  With  an  Electrolytic  Detector. 

be  one  half  that  of  the  battery.  This  device  thus  serves 
as  a  convenient  method  for  obtaining  very  minute  varia- 
tions in  the  detector  potential.  In  operation,  the  slider 
is  started  at  B  and  moved  along  the  potentiometer  until  a 
hissing  noise  is  heard  in  the  receivers.  This  indicates  that 
the  polarization  of  the  detector  is  being  broken  down. 
The  voltage  is  accordingly  reduced  until  this  noise  ceases, 
in  which  adjustment  the  detector  is  most  sensitive. 

342.  We  shall  later  see  that  the  phenomenon  of  recti- 
fication forms  the  basis  on  which  the  operation  of  many 
types  of  detectors  is  founded.  The  following  explanation 
will  serve  not  only  as  explanatory  of  the  operation  of  the 
electrolytic  detector  but  also  of  the  other  rectifying  de- 
tectors. 

212 


DETECTORS. 


[343 


343.  With  a  wave  length  of  600  meters,  the  frequency 
of  the  alternating  current  is  500,000  cycles,  as  we  have 
previously  seen.  In  the  operation  of  the  receiver,  the  an- 
tenna or  primary  circuit  is  first  adjusted  so  that  its  capa- 
city and  inductive  reactances  will  be  equal  at  that  fre- 
quency, when  there  will  be  a  maximum  current  flow  in 
the  circuit.  This  maximum  current  will  induce  a  maxi- 
mum potential  across  the  terminals  of  the  secondary  of  the 
receiving  transformer.  By  means 
of  the  variable  condenser  VC  of 
Fig.  105,  the  secondary  circuit- 
consisting  of  the  secondary  wind- 
ing and  the  variable  condenser- 
is  placed  in  a  state  of  resonance. 
This  is  exactly  analogous  to  the 
conditions  which  obtain  in  the 
secondary  circuit  of  the  transmit- 
ter. (See  paragraph  136.)  There 
is  thus  existent  a  maximum  po- 
tential across  the  terminals  of 
this  condenser.  This  potential, 
however,  is  alternating  and  of 
radio  frequency.  Consequently, 
the  current  which  it  sets  up  in  the 
telephone  circuit  will  be  of  so 
high  a  frequency  as  to  be  above 
the  limits  of  audibility.  The  problem,  then,  is 
duce  thrs  radio  frequency  to  an  audio  one. 


Fig.  1 06. 


to  re- 
T  in  Fig. 

106  shows  the  decaying  oscillating  potential  of  a  wave 
train  as  applied  across  the  terminals  of  the  detector. 
There  are  1,000  wave  trains  per  second  radiated  from  a 
transmitter  in  which  the  supply  current  is  of  500  cycles. 
That  is  to  say,  there  is  one  wave  train  emitted  per  alter- 
nation or  two  per  cycle.  Since  the  rectifying  detector  has 

213 


344]      ELEMENTS   OF  RADIOTELEGRAPHY. 

the  property  of  permitting  current  to  pass  thru  it  in  but 
one  direction,  the  lower  (or  upper)  half  of  each  cycle  is 
wiped  out,  leaving  a  pulsating  direct  potential  as  shown 
in  R,  Fig.  106.  This  potential  builds  up  or  is  stored  on  the 
condenser  SC  of  Fig.  105.  The  summation  or  integration 
of  these  direct  current  impulses  impressed  upon  the  con- 
denser is  discharged  as  shown  in  C  of  Fig.  106.  This 
fairly  high  potential  causes  a  complete  break  down  or  de- 
polarization of  the  detector  with  the  triggering  of  a  direct 
current  from  the  battery  E  thru  the  telephone  receivers, 
producing  a  click.  One  click  or  impulse  is  heard  per  wave 
train.  The  radio  frequency  of  500,000  cycles  per  second 
has  thus  been  reduced  to  an  audio  frequency  of  500  cycles. 
This  action  will  be  seen  tal>e  slightly  different  from  that 
of  the  crystal  detectors  to  be-desctibed  below,  in  that  the 
latter  have  no  local  battery  current  to  be  triggered  thru 
the  receivers. 

344.  Crystal  Rectifiers. -It  has  been  found  by  G.  W. 
Pierce  and  G.  W.  Pickard,  both  American  physicists  and 
engineers,  that  certain  crystalline  substances  exhibit  the 
property  of  unilateral  conductivity,  that  is  to  say,  they  are 
more  conductive  to  electric  currents  in  one  direction  than 
in  the  other.  Accordingly,  they  have  been  adapted  to  the 
needs  of  radiotelegraphy  as  rectifying  detectors,  Some  of 
the  common  crystal  rectifiers  are  the  elements  silicon,  tel- 
lurium, boron  and  arsenic,  and  sulphur  compounds  of  the 
elements  lead  (galena),  copper  and  iron  (bornite  and  chat- 
copyrite),  iron  (iron  pyrites  and  markasite),  and  molyb- 
denum (molybdenite).  Certain  oxides  are  also  in  use, 
namely,  —  those  of  zinc  (zincite),  copper  (cuprite)  and  lead 
(cerussite).  The  latter  detector  has  been  widely  used  by 
the  Marconi  Company.  The  crystals  are  mounted  in  metal 
cups,  retained  herein  by  a  solder  of  low  fusing;  tempera^ 

214 


DETECTORS.  [346 

ture  such  as  Wood's  metal.  (It  has  been  found  that  high 
temperatures  cause  them  to  lose  their  sensitiveness.)  A 
fine  contact,  such  as  the  point  of  a  spring  brass  or  gold 
wire,  is  mounted  so  as  to  bear  on  the  exposed  surface  of 
the  crystal.  The  wire  or  the  crystal,  or  both,  are  ar- 
ranged to  be  moved  with  respect  to  each  other,  as  all 
points  of  the  crystal  are  not  sensitive  and  a  careful  search 
must  be  made  to  ascertain  the  most  favorable  position. 
Occasionally,  two  crystals  are  arranged  to  be  used  together, 
a  point  of  zincite  being  used  with  either  bornite  or  chal- 
copyrite.  Such  a  combination  is  called  the  perikon  detec- 
tor. Similarly,  arsenic  compounds  are  often  used  in  con- 
junction with  silicon. 

345.  Carborundum,  a  carbide   of  silicon — an  artificial 
compound — may  also  be  used  as  a  rectifier,  but  requires 
a  small  constant  potential  across  its  terminals  to  be  most 
sensitive.     (When  so  employed,  the  diagram  of  Fig.  105 
is  used.)     With  the  average  crystal  rectifier,  a  light — but 
not  imperfect — contact  of  the  wire  on  the  crystal  gives  the 
best  results.     Carborundum,  however,  may  be  used  with 
a  rugged  point  bearing  on  it  under  considerable  pressure. 
Accordingly,  in  the  days  when  the  crystal  detector  was  the 
most  common  one  in  use,  the  carborundum  detector  en- 
joyed great  popularity  aboard  ship,  where  it  could  not  be 
jarred  out  of  adjustment  by  heavy  seas  nor  by  the  vibra- 
tions of  gunnery  exercises. 

346.  The  diagram  of  connections  used  with  the  crystal 
rectifiers  named  in  paragraph  344  is  shown  in  Fig.  107. 
The  arrangement  of  the  primary  and  secondary  circuits  is 
seen  to  be  similar  to  that  of  the  preceding  figures.     The 
detector  or  telephone  circuit,  however,  is  slightly  different. 
The  receivers  T  are  shunted  across  the  stopping  condenser 
SC,  which  is  in  series  with  the  detector  D.     As  shown  in  Fig. 

17  215 


347]       ELEMENTS   OF  RADIOTELEGRAPHY. 

106,  the  alternating  potential  T  is  applied  across  the  term- 
inals of  the  detector.  The  rectifying  properties  of  the  crystal 
permit  a  pulsating  direct  current  only  to  pass  thru  it,  charg- 
ing the  stopping  condenser  with  recurring  direct  current 


Fig.  107.     Crystal  Detector  Circuit. 

impulses  as  shown  in  R.  The  condenser  then  discharges 
thru  the  telephone  .receivers  once  for  the  wave  train.  It 
will  be  observed  that  while  there  is  but  one  discharge  of 
the  condenser  per  wave  train,  the  number  of  charges  it 
receives  depends  upon  the  damping  of  the  train.  For  high 
damping  there  are  fewer  waves  in  the  train,  and  hence 
fewer  charges,  than  for  low  damping  or  decrement. 

347.  While  the  crystal  detector  is  still  in  use,  it  has  been 
almost  completely  superseded — in  all  modern  installations 
—by  the  various  forms  of  valve  or  audion  detectors  to  be 
described  in  the  following  chapter. 

348.  Magnetic. — This    type    of    detector   preceded    the 
crystal  rectifier  historically,  but  due  to  the  similarity  be- 
tween the  latter  and  the  electrolytic  type,  it  was  considered 
advisable  to  discuss  them  successively.     The  magnetic  de- 

216 


DETECTORS. 


[348 


tector  was  the  invention  of  Prof.  E.  Rutherford,  but  was 
later  improved  by  Marconi  and  was  quite  widely  used  as 
a  detector  by  the  British  Marconi  Company.  It  is  operated 
on  the  principle  of  the  variation  in  hysteresis  of  the  iron 
core  of  an  electromagnet  when  it  is  subjected  to  high  fre- 
quency currents.  (See  paragraph  135.)  A  diagram  is 
shown  in  Fig.  108.  Two  horseshoe  magnets  are  placed 


Fig.  1 08.     Magnetic  Detector. 

with  their  like  poles  together  so  as  to  magnetize  a  bundle 
of  fine,  insulated,  iron  wires  revolved  by  the  pulleys 
A  and  B.  These  are  slowly  driven  by  clock  work  so  that 
the  velocity  of  the  endless  iron  band  is  about  four  feet 
per  minute.  Over  the  wire  where  the  state  of  magnetiza- 
tion is  greatest,  that  is  to  say,  directly  under  the  two  north 
poles  of  the  magnets,  is  wound  a  coil  of  insulated  wire 
which  is  connected  to  the  secondary  circuit  of  the  receiving 
transformer.  This  may  be  termed  the  radio  frequency 
coil  of  the  detector.  Over  this  coil,  and  insulated  there- 
from, is  wound  a  second  coil,  whose  terminals  are  connected 

217 


349]       ELEMENTS   OF  RADIOTELEGRAPHY. 

to  a  telephone  receiver,  which  we  may  term  the  telephone 
or  audio  frequency  coil.  As  the  iron  wires  are  driven 
at  a  constant  speed,  there  is  a  constant  magnetization 
of  the  wires  within  the  detector  coils.  There  is  also  a 
certain  lag  in  the  magnetization  of  the  wires  behind  the 
magnetizing  force,  that  is  to  say,  due  to  their  rotation,  they 
do  not  attain  maximum  magnetization  directly  under  the 
poles  of  the  magnets  but  at  a  short  distance  to  the  right — 
in  the  direction  of  the  rotation.  However,  since  this  lag 
or  hysteresis  is  constant,  no  induced  E.M.F.'s  are  set  up  in 
the  telephone  coil  and  the  receivers  are  silent.  When, 
however,  radio  frequency  oscillations  from  the  secondary 
circuit — due  to  incoming  signals — are  passed  thru  the  radio 
frequency  coil,  the  amount  of  hysteresis  is  reduced.  That 
is  to  say,  there  is  less  lag  of  the  magnetization  of  the  core 
behind  the  magnetizing  force — the  points  of  greatest  mag- 
netization are  shifted  to  the  left,  nearer  the  poles  of  the 
magnets.  This  change  in  the  magnetic  field  within  the 
telephone  coil  results  in  the  induction  of  a  transient  elec- 
tromotive force  therein  and  one  click  of  the  receiver  is 
heard  for  each  train  of  radio  frequency  oscillations  passing 
thru  the  detector.  This  type  of  detector  is  seen  to  be 
similar  to  the  other  types  previously  considered  in  that  the 
integral  effect  of  a  complete  train  of  waves  is  registered  as 
one  click  of  the  telephone  receivers.  The  radio  frequency 
current  of  the  incoming  signals  is  thus  reduced  to  an  audio 
one,  capable  of  detection  by  the  ear.  It  is  interesting  to  note 
that  the  ear  and  the  diaphragm  of  the  telephone  receiver 
have  similar  limitations  as  to  range  of  audibility  in  that 
even  if  the  one  were  able  to  respond  to  radio  frequencies, 
the  other  could  not. 

349.  This  type  of  detector  has  the  advantage  of  being 
extremely  rugged  and  reliable — the  operator  has  only  to 

218 


DETECTORS. 


[350 


assure  himself  of  the  rotation  of  the  iron  core  to  know  that 
the  detector  is  in  an  operative  condition.  It  has  the  dis- 
advantage, however,  of  being  rather  insensitive. 

350.  Tikker. — This  type  of  detector  has  been  noted  in 
paragraph  250.  The  types  of  detectors  previously  consid- 
ered have  operated  on  the  principle  of  producing  a  click  for 
each  train  of  oscillations.  With  the  Poulsen  arc  transmit- 
ter, or  any  other  type  employing  undamped  or  continuous 


Fig.  109.     Tikker  Receiving  Circuit. 

waves,  there  is  but  one  wave  train  emitted  for  each  depres- 
sion of  the  key.  Consequently,  when  the  detectors  noted 
above  are  used,  a  single  click  is  heard  at  the  beginning  of 
the  train  and  another  at  the  end.  It  becomes  necessary, 
therefore,  to  provide  means  at  the  receiver,  if  not  already 
provided  at  the  transmitter,  for  breaking  up  the  long 
wave  train  into  a  number  of  trains,  corresponding  to 
the  emission  of  some  1,000  trains  per  second  in  the  aver- 
age spark  transmitter.  A  device  originally  used  by  the 

219 


351]      ELEMENTS   OF  RADIOTELEGRAPHY. 

early  Poulsen  companies  was  a  make  and  break  circuit, 
similar  to  the  vibrator  of  an  induction  coil.  (See  para- 
graph 139.)  A  diagram  is  shown  in  Fig.  109.  The  make 
and  break  device  or  tikker  is  shown  at  T,  and  is  in  series 
with  the  telephone  receivers,  shunted  across  which  is  a 
large  condenser,  SC.  It  will  be  seen  that  Fig.  109  is  iden- 
tical with  Fig.  107,  so  far  as  the  position  of  the  respective 
detectors  is  concerned. 

351.  When  the  tikker  is  open  as  shown  in  the  figure, 
there  is  no  audio  circuit  connected  to  the  secondary  or  radio 
frequency  circuit  as  with  all  other  types  of  detectors.  Con- 
sequently, there  is  no  absorption  of  energy  from  the  latter 
circuit,  and  low  decrement  and  consequent  full  use  of  the 
principle  of  resonance  may  be  obtained  therein.  As  a 
result,  higher  potentials  than  are  usually  obtained  will  be 
resident  on  the  condenser  VC,  When  the  tikker  is  closed, 
the  condenser  SC,  which  is  very  much  larger  than  VC,  will 
receive  the  bulk  of  the  charge  which  was  on  FC,  and  dis- 
charge into  the  telephone  receivers,  producing  a  click.  The 
note  heard  in  the  receivers  is  that  of  the  frequency  of  the 
make  and  break  of  the  tikker  Besides  the  fact  that  the 
tikker  is  necessary  to  break  up  the  undamped  received  cur- 
rent into  a  series  of  wave  trains,  there  is  also  gained  the  in- 
creased efficiency  of  dissociating  the  secondary  and  detector 
circuits  so  as  to  permit  the  former  to  receive  a  large  charge 
before  transferring  it  to  the  latter.  The  action  here  will 
be  seen  to  be  analogous  to  that  of  the  quenched  gap,  in  that 
the  infinite  resistance  of  the  tikker  at  the  "break,"  and  the 
consequent  uncoupling  of  the  secondary  and  detector 
circuits  at  certain  instants,  results  in  increased  efficiency. 
The  superiority  of  the  tikker  may  even  be  demonstrated  in 
reception  from  spark  sets.  The  author  has  found  that 
greater  distances  can  be  covered  with  this  type  of  de- 
tector than  with  the  best  crystal,  altho  the  tone  of  signals 

220 


DETECTORS. 


[354 


is  rough  and,  for  other  than  high  frequency  spark  sets, 
quite  difficult  to  read. 

352.  The   tikker   has   the    disadvantage,   however,   of 
not  being  able  to  properly  time  its  make  and  break  to 
the  periods    of    charge  and   discharge    of    the  two  con- 
densers concerned.     Consequently,  the  note  heard  in  the 
receivers  is  not  musical,  rather — a  smooth,  hissing  tone. 

353.  L.  W.  Austin  has  designed  a  form  of  tikker  which 
employs  a  rotary  metal  disk  on  the  periphery  of  which  is 
cut  a  light  groove.     A  fine  spring  steel  wire  rests  therein, 
as  shown  in  Fig.  110.     Light  pressure  of  the  wire  is  main- 
tained, and  the  "chattering"  of  the  wire  on  the  wheel  makes 
and  breaks  the  circuit.     It  has  been  found  that  best  re- 
sults are  obtained  when  the  wheel  is  revolved  so  as  to 
run   against  the  point  of   the   wire.      More  pronounced 
"chattering"  and  more  decisive  makes  and  breaks  of  the 
circuit  result,  with  clearer  and  louder  signals. 


Fig.  no.     Tikker. 

354.  It  has  been  found  advantageous  to  mount  the  wheel 
of  the  rotary  tikker  on  the  shaft  of  an  A.C.  induction  motor, 
rather  than  on  that  of  a  D.C.  motor,  for  the  induction  in 
the  telephone  receivers  from  sparking  at  the  commutator 

221 


355]       ELEMENTS   OF  RADIOTELEGRAPHY. 

of  the  latter  often  interferes  with  the  reception  of  signals. 
This  type  of  tikker  was  widely  used  in  this  country  for  un- 
damped wave  reception  prior  to  the  advent  of  the  valve 
detectors  to  be  discussed  later,  was  absolutely  reliable,  and 
proved  generally  satisfactory. 

355.  While  the  subject  of  evacuated  valves  should  prop- 
erly be  covered  in  this  section  on  detectors,  it  is  considered 
advisable  to  devote  a  separate  chapter  to  their  study.     Ac- 
cordingly, the  remainder  of  this  chapter  will  be  given  to  a 
consideration  of  the  various  forms  of  modern  receivers. 

L. 
MODERN  RECEIVERS. 

356.  So  far  as  receivers  only  are  concerned,  and  irrespec- 
tive of  the  detectors  employed,  we  may  class  them  as  we 
classed  transmitters — into  single  or  double  circuit  sets. 
We  shall  find  that  they  exhibit  the  same  qualities  as  trans- 
mitters of  the  same  classification.     Thus,  as  we  have  ob- 
served in  the  early  part  of  this  chapter,  the  untuned  sec- 
ondary type  of  receiver,  in  which  non-oscillatory  currents 
occur  in  the  secondary  circuit — due  chiefly  to  its  high  re- 
sistance— is  similar  to  the  single  circuit  transmitter.     To 
receive  any  desired  wave    length   it    is    only  necessary 
to  adjust  the  antenna  or  primary  circuit  to  resonance,  just 
as  with  the  single  circuit  transmitter  it  is  only  required 
to  tune  the  antenna  for  the  radiation  of  any  desired  tune. 
Accordingly,  for  the  various  forms  of  high  resistance  de- 
tectors operated  on  the  current  principle,  i.e.,  crystal  recti- 
fiers, the  untuned  secondary  receiver  is  the  most  efficient, 
both  in  facility  of  adjustment  and  in  intensity  of  signals. 

Fig.  Ill  shows  the  scheme  of  connections  for  the  modern, 
untuned  secondary  receiver.  It  will  be  noted  that  this  is 
similar  to  Fig.  102  with  the  exception  that  the  coherer  and 

222 


MODERN  RECEIVERS. 


[357 


local  battery  have  together  been  replaced  by  the  crystal 
detector.     The  inherent  high  resistance  of  the  crystal  de- 


Fig,  in.    Untuned  Secondary  Receiver. 

tector  and  the  large  capacity  of  the  stopping  condenser 
serve  to  make  the  detector  circuit  an  aperiodic  one.  This 
scheme  of  connections  is  used  by  those  systems  enum- 
erated in  paragraph  334.  Occasionally,  however,  a  small 
constant  potential  may  be  applied  across  the  terminals  of 
the  crystal  detector  as  with  the  carborundum  type — to 
secure  greater  sensitivity,  but  this  in  nowise  affects  the 
aperiodic  nature  of  the  current  flow  in  the  detector  circuit. 

357.  We  have  seen,  however,  that  the  coherer  and  the 
electrolytic  detector— and  we  shall  find  that  the  valve 
detectors  are  similar  in  this  respect — require  a  large 
potential  for  their  successful  operation.  It  may  be  stated 
that  any  detector  utilizing  the  trigger  principle  is  so 
operated.  Accordingly,  the  use  of  tuned  coupled  circuits 
in  receiving  becomes  necessary  for  securing  a  high 
secondary  potential,  by  utilization  of  the  principle  of  res- 
onance in  that  circuit  for  the  building  up  of  a  high  poten- 

223 


358]       ELEMENTS   OF  RADIOTELEGRAPHY. 

tial  across  the  secondary  condenser.  (See  paragraph  136.) 
As  previously  explained,  this  step  has  its  disadvantages  in 
requiring  a  multiplicity  of  adjustments  in  both  primary  and 
secondary  circuits  in  order  that  the  coupling  and  their  re- 
spective wave  lengths  be  in  proper  accord  for  securing 
maximum  selectivity — sharpness  of  tuning — and  intensity 
of  signals.  (For  very  great  selectivity,  the  Marconi  Com- 
pany has  added  an  intermediate  oscillating  circuit  consist- 
ing of  two  inductances  in  series,  one  coupled  to  the  pri- 
mary of  the  receiving  transformer,  the  other  to  the  sec- 
ondary, with  a  variable  condenser  either  in  series  with  the 
two  or  in  shunt.  This  involves  the  use  of  three  oscillating 
circuits  — primary,  secondary,  and  intermediate — with  con- 
sequent increase  in  the  labor  of  tuning  adjustments.) 

LI. 
RECEIVING  TRANSFORMERS.1 

358.  The  inductances  comprising  the  antenna  loading 
inductance  and  the  primary  and  secondary  windings  of  the 
receiving  transformer  are  usually  wound  with  silk  or  enamel 
insulated  wires  of  small  diameter.  They  are  provided 
with  taps  taken  off  at  regular  intervals  to  a  selective  switch 
in  order  that  variation  of  the  inductance  may  be  effected. 
The  variable  condenser  Ci  of  Fig.  103  serves  to  secure  an 
exact  adjustment  of  the  wave  length  of  the  antenna  circuit 
after  an  approximate  adjustment  has  been  made  with  the 
tapped  inductances.  The  use  of  litzendraht — for  the  same 

1  In  place  of  the  receiving  transformers  described  below,  increasing 
use  is  being  made  of  capacity  or  static  coupling  between  the  primary 
and  secondary  circuits  of  receivers,  especially  of  the  electron  tube 
type.  Variable  condensers  are  used  for  this  work,  and  the  mode  of 
coupling  is  similar  to  that  shown  in  Fig.  16,  c.  Here,  LI  would  repre- 
sent the  antenna  circuit  constants  and  L>  those  of  the  secondary  circuit. 
C,  in  this  case,  would  be  variable. 

224 


RECEIVING    TRANSFORMERS.  [359 

purpose  as  in  the  transmitting  apparatus — is  common. 
With  the  low — and  in  the  case  of  undamped  waves,  zero- 
decrements  of  modern  transmitters,  it  becomes  necessary 
to  have  the  resistance  of  the  receiver  circuits  as  low  as  pos- 
sible in  order  that  the  receiver  decrement  may  be  reduced 
and  the  selectivity  increased.  (See  paragraphs  226  and  273.) 
The  construction  of  the  windings  of  the  receiving  trans- 
former is  similar  to  that  used  in  the  antenna  coupler  of 
the  transmitter  in  that  the  windings  are  either  cylin- 
drical, and  arranged  to  be  telescoped  (for  close  coupling), 
or  are  spirally  wound,  and  arranged  to  be  rotated  with 
respect  to  each  other. 

359.  Since  these  inductances  are  quite  compact  and 
very  closely  wound,  it  becomes  necessary  to  employ  cer- 
tain artifices  for  the  elimination  of  what  is  termed  distrib- 
uted capacity.  Across  an  inductance,  there  exists  a  po- 
tential due  to  its  reactance.  (See  paragraph  136.)  This 
difference  of  potential  between  the  terminals  of  the  coil, 

(5)  ®  (jo)  ®  ®  0      ©00®®® 
000000      00000® 


(a)  0>) 

Fig.  112. 

and  in  particular — between  adjacent  turns  of  wire  compris- 
ing it — sets  up  lines  of  static  force  within  the  coil.  These 
lines  of  static  force  will  be  the  greatest  for  a  minimum  sep- 
aration of  the  turns  across  which  there  exists  the  greatest 
potential.  If,  as  is  sometimes  the  case,  it  becomes  neces- 
sary to  use  more  than  one  layer  in  the  winding  of  an  in- 
ductance for  a  receiver,  care  should  be  taken  to  bank  the 

225 


360]       ELEMENTS   OF  RADIOTELEGRAPHY. 

turns.  Fig.  112  illustrates  the  two  methods  of  winding  a 
multiple  layer  coil.  In  a,  as  is  customary,  the  winding  is 
started  at  turn  1  and  is  continued  along  the  core  to  its 
other  end  at  turn  6.  Here  the  winding  begins  its  second 
layer  and  turn  7  is  wound  over  turn  6  of  the  first  layer.  At 
the  end  of  the  second  layer  we  find  turn  12  directly  over  turn 
1.  These  two  turns  have  thus  the  difference  of  potential 
of  12  turns  existing  across  the  narrow  space  between 
them.  In  the  banked  winding  shown  in  b,  turn  2  is 
wound  directly  over  turn  1,  turn  3  on  the  core,  turn  4  over 
turn  3,  and  turn  5  on  the  core  again.  Thus,  in  the  latter 
mode  of  winding,  the  greatest  difference  of  potential  exist- 
ing between  two  adjacent  turns  is  that  across  three  turns 
of  wire.  This  does  not  increase  no  matter  how  long  the 
core  be  made,  and  the  inductance  is  the  same  as  with  the 
form  of  winding  of  a.  The  distributed  capacity  is  thus  re- 
duced by  the  use  of  banked  turns,  and  this  forms  of  wind- 
ing is  universally  employed  in  the  manufacture  of  multiple 
layer  coils  for  radio  purposes. 

360.  A  brief  consideration  of  the  effects  of  distributed 
capacity  will  be  found  of  interest.  An  inductance  contain- 
ing distributed  capacity  may  be  represented  as  shunted 
by  a  condenser  of  equal  capacity — as  shown  at  the  top  of 
Fig.  113.  Since  this  forms  an  oscillating  circuit,  it  gives  the 
coil  a  definite  period  or  wave  length  of  its  own.  If  it  be  de- 
sired to  use  this  coil  as  the  secondary  of  a  receiving  trans- 
former for  an  untuned  secondary  receiver,  the  secondary 
circuit  will  not  be  aperiodic  as  it  should,  but  will  have  a  defi- 
nite wave  length.  When  the  wave  length  of  the  antenna  cir- 
cuit is  equal  to  that  of  the  secondary  coil,  signals  may  be 
efficiently  received  on  that  tune,  but  should  the  antenna 
circuit  be  tuned  to  some  other  wave  length  on  which  in- 

226 


RECEIVING   CONDENSERS. 


[361 


coming  signals  may  be  received,  the  two  circuits  will  now 
be  out  of  resonance,  with  consequent  reduced  efficiency. 
Further,  if  only  part  of  the 
winding  of  a  coil  with  dis- 
tributed capacity  is  in 
use,  the  remainder  of  the 
coil  will  set  up  a  small 
oscillating  circuit  by  it- 
self, as  shown  in  the  lower 
part  of  Fig.  1 13,  which  will 
react  upon  that  part  of  the 
coil  which  is  employed, 
with  consequent  distortion 
of  the  tuning.  If  the  sec- 
ondary winding  of  the 
step-up  transformer  of  a  FiS-  IJ3- 

transmitter  contains  dis- 
tributed capacity,  as  may  be  quite  often  the  case,  the 
oscillating  circuit  set  up  thereby  may  be  in  resonance,  or 
approximately  so,  w  th  the  radio  frequency  currents  in  the 
gap  circuit.  This  will  permit  radio  frequency  currents  of 
high  potential  to  make  their  way  into  the  transformer  and 
possibly  damage  it.  (See  paragraph  185.) 


LII. 
RECEIVING   CONDENSERS. 

361.  Under  this  heading,  will  be  considered  fixed  and 
variable  condensers.  We  have  seen  that  most  receivers 
comprise  a  fixed  condenser,  labelled  SC — stopping  con- 
denser. These  condensers  are  usually  made  of  sheets  of 
tinfoil,  separated  from  each  other  by  paraffined  paper  (the 
potentials  are  very  low)  and  rolled  into  a  compact  mass. 
Occasionally,  small  mica  condensers  are  employed  for  this 

227 


362]       ELEMENTS   OF  RADIOTELEGRAPHY. 

purpose  (see  paragraph  152),  and  other  types  of  dielectric 
may  be  used.  The  fixed  condensers  in  receiving  circuits 
have  fairly  large  capacity,  averaging  several  tenths  of  a 
microfarad. 

362.  The  variable  condensers  in  receiver  circuits  com- 
monly employ  air  as  their  dielectric,  altho  oil  may  be  used 
to  increase  the  capacity,  and  the  Marconi  Company  has 
even  employed  thin  hard  rubber  sheets.  Variable  con- 
densers are  usually  built  in  the  rotary  type,  consisting  of 


0  O) 

Fig.  114.     Variable  Condenser. 

semicircular  stationary  and  revolving  metal  plates.  (See 
Fig.  114.)  The  stationary  plates  are  built  up  in  one  group, 
and  the  rotary  plates  are  mounted  on  a  shaft,  revolved  by 
an  insulated  handle.  The  plates  interleave,  as  in  the  Fes- 
senden  compressed  air  transmitting  condenser  previously 
described,  and  the  capacity  of  the  condenser  may  be  very 
minutely  varied  as  the  size  of  the  opposing  areas  is  changed. 
Theoretically,  the  capacity  of  the  condenser  is  zero  in  the 
position  shown  in  a  of  the  figure.  As  a  matter  of  fact,  how- 
ever, there  is  an  appreciable  capacity  existing  between  the 
edges  of  the  plates,  even  tho  they  be  not  interleaved. 
The  maximum  capacity  of  such  condensers  averages  from 
0.001  mf.  to  0.005  mf.,  according  to  the  separation  and 
number  of  the  plates,  and  the  nature  of  the  dielectric 
used. 

228 


18 


TELEPHONE  RECEIVERS.  [365 

363.  A  sliding  type  of  variable  condenser  used  by  the 
Marconi  Company  consists  of  two  metallic  tubes  of  differ- 
ent diameter,  arranged  so  as  to  be  telescoped,  but  insulated 
from  each  other.     The  degree  of  telescoping  determines 
the  capacity.     This  type  is  only  used,  however,  for  very 
small  capacities,  as  there  are — in  effect — but  two  plates. 
Another  type  of  sliding  condenser  consists  of  a  series  of 
movable  and  of  stationary  plates  mounted  in  a  rack,  the 
movable  plates  sliding  in  grooves  and  cleats  between  the 
stationary  ones. 

364.  A  variable  condenser  may  be  used  in  various  parts 
of  the  receiver  circuit.     Shunted  across  the  inductances  in 
the  antenna,  it  serves  to  increase  the  wave  length.     Shunted 
across  the  secondary  coil  of  the  receiving  transformer,  it 
enables  the  operator  to  secure  a  fine  variation  of  the  wave 
length  in  the  secondary  circuit.     In  series  with  the   an- 
tenna, it  reduces  the  antenna  wave  length  so  as  to  make 
it  possible   to  receive   signals  on  a  wave   length  shorter 
than  the  fundamental.     (See  paragraphs  171  and  286.) 

LIII. 

TELEPHONE   RECEIVERS. 

365.  In  the   pioneer   days   of  the   art,  coincident   with 
the  use  of  the  coherer  as  a  detector,  Morse  sounders  and 
ink  recorders  were  variously  used  as  the  current  indicating 
devices.     In  recent  years,  other  types  of  visual  recorders 
have  been  employed — chiefly  in  conjunction  with  auto- 
matic, rapid  transmitters.     One  device,  the  telegraphone 
—invented  by  Poulsen— makes  use  of  the  magnetization 
of  a  steel  wire  for  the  recording  of  the  received  impulses. 
With  proper  amplification,  signals  may  also  be  registered 

229 


366]      ELEMENTS   OF  RADIOTELEGRAPHY. 


on  wax  phonograph  records,  or  on  a  strip  of  sensitized 
photographic  paper  by  the  deflection  of  a  beam  of  light  re- 
flected from  a  small  mirror  on  the  moving  member  of  a 
galvanometer. 

366.  For  all  ordinary  reception,  however,  telephone  re- 
ceivers are  commonly  used.  These  are  of  the  watch  case 
type,  mounted  on  head  bands  and  worn 
with  a  receiver  over  each  ear.  The  tele- 
phone receiver  consists  of  a  steel  per- 
manent magnet  carrying  a  soft  iron  core 
at  one  end  over  which  is  wound  a  coil  of 
wire  thru  which  the  current  to  be  detected 
is  passed.  Very  small  currents  produce 
changes  in  the  magnetic  field  set  up  by  the 
magnet  which  produce  movements  in  the 
iron  diaphragm  which  in  turn  are  detected 
by  the  ear.  Fig.  115  shows  the  single 
pole  type  of  hand  receiver  while  Fig.  116 
shows  the  bipolar  arrangement  applicable 
to  either  the  hand  or  head  (watch  case)  type;  this 
employs  a  U-shape  magnet  in  place  of 
the  bar  magnet  of  the  other  receiver. 
The  bar  magnet  has  the  advantage  of 
applying  the  magnetic  impulses  of  the 
electromagnet  in  the  center  of  the  di- 
aphragm where  the  greatest  movement 
can  be  imparted  to  it.  On  the  other 
hand,  the  reluctance  of  its  magnetic 
circuit  is  very  high  (see  paragraph  133), 
since  the  lines  of  force  have  to  flow 
from  the  North  pole  to  the  diaphragm  and  back  to  the 
South  pole  thru  the  air.  In  the  type  shown  in  the  latter 
figure,  the  reluctance  of  the  circuit  is  greatly  reduced, 

230 


Fig.  115. 


Fig.  1 1 6. 


TELEPHONE  RECEIVERS. 


[367 


since  the  lines  of  force  flow  from  the  North  pole  thru 
the  diaphragm  back  to  the  South  pole,  with  only  the  inter- 
vening air  space  between  the  diaphragm  and  the  poles. 
This  type  of  receiver  has  the  disadvantage,  however,  of 
not  applying  its  magnetic  impulses  to  the  diaphragm  at 
its  center,  where  the  greatest  mechanical  movement  can 
be  set  up.  A  new  type  of  receiver,  shown  in  Fig.  117, 


Fig.  117. 

combines  the  favorable  points  of  the  other  types.  A  mica 
diaphragm  is  employed,  to  which  is  fastened  a  lever  AB, 
connected  to  the  thin  iron  lever  BC.  The  armature  EC 
lies  directly  in  the  path  of  the  flux  between  the  two  poles 
of  the  magnet  so  that  the  reluctance  is  even  lower  than 
with  the  watch  case  receiver.  The  mechanical  impulses 
are  communicated  directly  to  the  center  of  the  diaphragm. 
Accordingly,  this  type  of  receiver  is  by  far  the  most  sensi- 
tive on  the  market. 

367.  The  magnetizing  force  which  sets  up  lines  of  flux 
in  a  magnetic  circuit  of  given  reluctance  corresponds  to 
the  electromotive  force  which  causes  a  current  to  flow  in  a 
circuit  of  given  resistance.  It  is  termed  magnetomotive 

231 


367]       ELEMENTS   OF  RADIOTELEGRAPHY. 

force,  M.M  F.,  and  is  given  by  the  formula 

M  =  1.25772/,  (63) 

where  M  is  the  magnetomotive  force,  n  represents  the  num- 
ber of  turns  of  the  inductance  or  electromagnet,  and  /  is 
the  current  flowing  therein.  The  expression  nl  is  termed 
ampere-turns )  and  is  considered  as  a  single  quantity.  For  a 
maximum  response  of  a  telephone  receiver,  it  is  desired  that 
nl  be  as  large  as  possible.  There  is  a  definite  limit  to  this 
quantity,  however,  for  if  n  be  made  too  large,  the  resist- 
ance of  the  receiver  will  be  so  high  that  a  reasonable  E.M.F. 
will  produce  but  a  small  /.  Accordingly,  in  the  design  of 
telephone  receivers,  nl  is  given  a  certain  maximum  limit. 
With  such  detectors  as  the  electrolytic,  crystal  and  valve, 
their  inherent  high  resistances  limit  the  flow  of  current  in 
the  detector  or  telephone  circuit  to  a  very  small  quantity. 
Consequently  with  a  small  value  of  7,  it  is  necessary  to  have 
a  large  value  of  n.  Receivers  for  use  in  conjunction  with 
high  resistance  detectors  accordingly  are  wound  with  a 
great  number  of  turns  of  wire.  In  order  that  the  winding 
may  be  encompassed  in  as  small  a  space  as  possible,  for 
both  mechanical  and  electrical  reasons,  the  size  of  the  wire 
must  be  very  small,  from  36  to  50  B.  &  S.  gauge.  Such  wire 
gives  a  high  resistance  to  the  telephone  receiver.  It  is 
customary  for  a  head  set  to  have  a  resistance  of  from  1,000 
to  1,600  ohms  per  receiver.  Since  the  two  receivers  are 
in  series,  the  total  resistance  runs  as  high  as  3,200  ohms 
per  set.  With  the  magnetic  and  tikker  detectors,  on  the 
other  hand,  the  resistance  is  quite  low — consequently,  for 
the  increased  value  of  current  in  the  telephone  circuit,  the 
number  of  turns  may  be  reduced.  The  telephone  receivers 
for  use  with  this  type  of  detector  need  have  a  resistance  of 
but  150  ohms  per  set.  While  it  is  customary  to  rate  tele- 
phone receivers  according  to  their  resistances,  since  they 

232 


AUDIBILITY  MEASUREMENTS.  [360 

offer  a  ready  means  of  comparison,  it  should  be  borne  in 
mind  that  the  resistance,  except  as  it  is  indicative  of  the 
number  of  turns  of  copper  wire  on  the  electromagnet,  plays 
no  part  in  the  sensitivity  of  a  receiver. 

368.  Various  forms  of  electro-mechanical  amplifiers  have 
been  used  in  conjunction  with  telephone  receivers  for  in- 
creasing the  intensity  of  received  signals.    Two  notable 
examples  are  the  Brown  relay  used  by  the  Marconi  Com- 
pany, and  a  polarized  relay  employed  by  the  Telefunken 
Company.     The  latter  company  also  made  use  of  a  sound 
amplifier  operated  on  the  principle  of  audio  resonance— 
i.e.,  responding  only  to  sparks  of  definite  frequency. 

LIV. 
AUDIBILITY  MEASUREMENTS. 

369.  The  taking  of  data  on  the  strength  of  received  signals 
constitutes  what  is  termed   an  audibility  measurement. 
Unit  audibility  is  taken  as  that  strength  of  signal  at  which 
dots  may  just  be  distinguished  from  dashes.     If  a  variable 
shunt  impedance  be  connected  across  the  telephone  re- 
ceivers, the  strength  of  signals  may  be  reduced  so  that 
they  are   just  audible.     In  this  condition,  the  intensity  of 
signals  —  measured  as  uso  many  times  audibility"  —  is  given 
by  the  formula 

•-,  (64) 


where  a  is  the  audibility  factor  in  "number  of  times  audi- 
bility," R  is  the  impedance  of  the  telephone  receivers  to 
the  received  signals,  and  S  is  the  impedance  of  the  shunt 
under  the  same  conditions.  Audibility  boxes  for  use  with 
telephone  receivers  of  definite  resistance  and  certain  type 
are  supplied.  They  consist  of  a  resistance,  tapped  to  sev- 

233 


370]      ELEMENTS   OF  RADIOTELEGRAPHY. 

eral  buttons  which  are  graduated  according  to  the  solution 
of  equation  (64).  Audibilities  may  thus  be  read  directly — 
without  computation.  This  method  is  notoriously  inac- 
curate, nevertheless  it  serves  as  a  ready  means  of  compar- 
ison of  signal  strengths,  and  is  widely  used. 

370.  The  following  method  for  audibility  measurements, 
termed  the  non-resonant  method,  is  in  use  in  the  Bureau 
of  Standards,  and  is  commonly  used  by  the  Navy.     With 
the  modern  valve  detector  receiver,  so  great  a  reinforce- 
ment of  signals  may  be  obtained  by  careful  adjustment  of 
the  coupling  and  tune  of  the  primary  and  secondary  cir- 
cuits, that  the  strength  of  received  signals  is  more  often  a 
matter  of  the  operator's  skill  than  intensity  of  received  cur- 
rents.    Accordingly,  it  is  desired  to  employ  a  method  which 
will  give  fairly  uniform  results,  irrespective  of  the  observer. 
Signals  are  first  received  with  the  primary  and  secondary 
circuits  fairly  closely  coupled.     The  coupling  is  then  weak- 
ened, with  careful  readjustment  of  both  primary  and  sec- 
ondary circuits  so  as  to  keep  them  in  accord.     At  the  weak- 
est point  of  coupling  obtainable,  the  circuits  are  finally  ad- 
justed for  maximum  intensity  of  signals.     The  coupling  is 
now  increased,  but  the  primary  tune  is  left  unchanged.    The 
secondary  tune  is  now  varied  with  different  degrees  of  coup- 
ling until  the  greatest  intensity  of  signals  is  obtained.     The 
primary  tune,  however,  is  not  changed  from  its  adjustment 
at  the  point  of  weakest  coupling.     The  reading  of  the  audi- 
bility meter  is  now  taken. 

LV. 
HARMONIC   OSCILLATION   OF  RECEIVERS. 

371.  We  have  observed  in  paragraph  278  that  a  cord 
may  be  excited  to  vibration  by  impulses  which  are  harmon- 
ics of  its  fundamental  frequency.     Similarly,  a  receiver 
when  tuned  to  a  certain  wave  length,  will  respond  to  oscil- 

234 


HARMONIC  OSCILLATION  OF  RECEIVERS.  [372 

lations  which  are  harmonics  of  its  own  tune.  Thus,  a 
receiver  adjusted  to  resonance  at  a  wave  length  of  15,000 
meters  will  respond  to  signals  on  a  tune  of  approximately 
5,000  meters,  its  third  harmonic.  (Only  an  oscillating 
circuit  of  distributed  inductance  and  capacity,  such  as  the 
unloaded  antenna  considered  in  the  preceding  chapter, 
will  have  its  fundamental  wave  length  an  exact  multiple  of 
its  harmonics.)  The  receiver  is  thus  oscillating  harmon- 
ically, and  unless  a  search  be  made  of  its  lower  range  so  as 
to  detect  the  presence  of  the  same  station  on  its  true  re- 
ceived tune,  the  operator  may  be  led  to  believe  that  he  is 
receiving  signals  on  the  fundamental  oscillation  of  his  re- 
ceiver instead  of  on  an  harmonic.  It  should  be  borne  in 
mind  that  it  is  not  an  harmonic  of  the  transmitter  which  is 
being  received.  Instead  its  fundamental  oscillation  is  pro- 
ducing an  harmonic  oscillation  of  the  receiver. 

372.  To  reiterate — in  order  to  obviate  the  error  occa- 
sionally caused  by  assuming  that  signals  received  at  a 
sharply  defined  tune  on  a  long  wave  length  adjustment  of 
the  receiver  are  emanating  from  a  long  wave  length  trans- 
mitter when,  as  a  matter  of  fact,  they  may  be  coming  from 
a  station  adjusted  approximately  to  one  third  or  one  fifth 
of  the  receiver  tune,  a  careful  search  should  be  made  on 
shorter  wave  lengths  so  as  to  obtain  the  true  wave  length 
of  the  transmitter  as  determined  by  the  fundamental  oscil- 
lation of  the  receiver. 


235 


CHAPTER  ELEVEN. 

LVI. 
THE  EDISON  EFFECT. 

373.  In  the  study  of  the  action  of  the  various  forms  of 
electron    tubes  or  valve  detectors,  which    are  variously 
termed  auctions,  electron  relays,  kenotrons,  pliotrons,  dy- 
natrons,  oscillions,  thermotrons  and  what-not,  according 
to  the  caprice  of  their  respective  inventors,  a  brief  consid- 
eration of  the  so-called  Edison  effect  will  be  found  of  in- 
terest. 

374.  In   1890,   or  thereabouts,   the   American  inventor 
Thomas  A.  Edison  discovered  an  interesting  phenomenon 
which  was  observed  in  the  manufacture  of  the  metal  fila- 
ment electric  lamp  which  he  designed.     When  such  a  fila- 
ment is  heated  to  incandescence,  it  radiates  electrons — 
negative  ions.     (See  paragraph  94.)     It  is  quite  generally 
believed  that  the  flow  of  current  in  a  metallic  conductor  is 
similar  to  that  in  an  electrolyte — that  is  to  say,  takes  place 
by  the  flow  of  electrons  or  ions,  actuated  by  an  electromo- 
tive force  applied  at  the  terminals  of  the  circuit.    When 
this  E.M.F.  is  not  impressed  upon  the  circuit,  the  electrons 
or  negative  corpuscles  within  the  metal  are  moving  about 
in  an  irregular  fashion,  colliding  with  and  bounding  off 
from  the  atoms  of  the  metal  with  which  they  come  in  con- 
tact.   We  have  observed  in  paragraph  97  that  the  molec- 
ular activity  of  a  gas  increases  as  it  is  heated.     Similarly, 
the  velocity  of  these  electrons  within  the  metal  increases 
as  its  temperature  is  raised.     Ordinarily,  these  moving 
electrons  do  not  become  detached  from  the  metal  at  its 

236 


THE  EDISON  EFFECT.  [376 

surface,  due  to  an  electrical  restraining  force  similar  to 
that  present  on  the  surface  of  a  liquid  which  is  termed  sur- 
face tension.  As  the  metal  is  heated  to  a  red  or  incandes- 
cent stage,  however,  the  "surface  tension"  is  not  sufficient 
to  restrain  the  electrons,  which  have  now  acquired  a  high 
velocity,  and  they  are  given  off  into  space.  This  is  similar 
to  the  evaporation  of  molecular  particles  of  a  liquid,  which 
process  is  facilitated  by  heat. 

375.  In  the  ordinary  electric  lamp,  the  radiation  of  nega- 
tive electrons  imparts  a  negative  space  charge  to  the  area 
surrounding  the  filament,  that  is  to  say,  the  space  in  the 
immediate  vicinity  of  the  filament  acquires  a  negative  charge 
just  as  tho  it  were  a  solid  body.     This  negative  charge 
tends  to  repel  the  like  charge  of  the  electrons  (see  para- 
graph 92),  with  the  result  that  most  of  them  are  driven 
back  into  the  filament  again.     Or  we  may  say  that  in  re- 
moving an  accumulation  of  negative  charges  from  the  fila- 
ment in  the  radiation  of  the  electrons,  we  are  leaving  it 
positively    charged.     This    preponderance    of    a    positive 
charge  thereon,  tends  to  attract  the  negatively  charged 
electrons   so  that  many  of  them  will  return  to  the  fila- 
ment.    We  shall  later  observe,  however,  in  a  consider- 
ation of  the  Fleming  valve,  that  some  of  the  electrons  radi- 
ated are  not  returned  to  the  filament,  and  their  presence  in 
the  space  surrounding  it  may  prove  of  value. 

376.  The  radiation  of  electrons  from  an  incandescent 
filament  may  be  easily  demonstrated.     If  an  additional 
member  or  electrode  be  introduced  into  the  electric  lamp, 
it  will  receive  a  negative  charge  from  those  electrons  which 
are  not  returned  to  the  filament.     This  may  be  shown  ex- 
perimentally by  connecting  one  terminal  of  a  galvanometer 
to  the  wing  or  plate — as  the  additional  electrode  is  termed 
—and  the  other  terminal  to  the  negative  terminal  of  the 

237 


377]      ELEMENTS   OF  RADIOTELEGRAPHY. 

lamp  filament.  With  this  connection,  the  lighted  filament 
will  produce  only  a  slight  deflection  of  the  galvanometer 
—if  any.  If  the  lead  from  the  galvanometer  now  be 
changed  from  the  negative  to  the  positive  side  of  the  fila- 
ment, a  very  pronounced  deflection  of  the  instrument  will 
occur.  This  will  indicate  a  difference  of  potential  between 
the  plate  and  that  side  of  the  filament  to  which  the 
galvanometer  is  connected.  Since  there  is  a  difference 
in  potential,  the  wing  must  have  received  a  charge  opposite 
in  sign  to  that  of  the  positive  terminal  of  the  filament,  and 
hence  a  negative  charge. 

377.  From  the  fact  that  the  filament  emits  only  negative 
ions,  it  is  obvious  that  a  rectifying  characteristic  must  be 
one  of  the  accompaniments  of  the  Edison  effect.     A  flow 
of  negative  ions  from  the  filament  to  the  wing  is  equivalent 
to  a  flow  of  positive  ions  from  the  wing  to  the  filament. 
Hence,  we  see  that  we   can  pass  a  much  greater  cur- 
rent from  the  plate  to  the  filament,  with  the  flow  of  positive 
ions,  than  from  the  filament  to  the  plate,  against  the  flow 
of  positive  ions. 

378.  The  Fleming  valve  was  invented  by  J.  A.  Fleming 
of  London  in  1904.     He  discovered,  as  a  natural  sequence 
to  his  investigation  of  the  Edison  effect,  that  the  phenom- 
enon of  unilateral  conductivity  accompanying  the  Edison 
effect  could  be  utilized  in  the  construction  of  a  rectifying 
detector  of  radio  frequency  currents.     His  valve  consisted 
of  a  lamp  with  a  carbon  filament,  metal  (tungsten)  ones 
were  introduced  later,  and  a  sealed-in  plate  or  wing.     A 
diagram  is  shown  in  Fig.  118,  where  L  represents  the  usual 
receiving  transformer,  C  the  secondary  condenser,  SC  the 
stopping  condenser,  T  the  telephone  receivers,  V  the  valve, 
consisting  of  filament  F  and  plate  P,  A  the  filament  battery 
and  R  a  small  variable  resistance  for  regulating  the  flow 

238 


ELECTRON  TUBE   DETECTORS. 


[379 


of  the  filament  current.  The  operation  of  the  Fleming 
valve  is  similar  to  that  of  the  various  crystal  detectors  in 
that  its  rectification  properties  permit  a  series  of  direct 


Fig.  1 1 8.     Fleming  Valve. 

current  impulses  to  collect  upon  the  stopping  condenser 
which  discharges  into  the  telephone  receivers  once  per 
wave  train. 

LVII. 

ELECTRON  TUBE   DETECTORS. 

379.  To  E.  H.  Armstrong,  of  New  York,  is  due  much  of 
our  modern  conception  of  the  action  of  electron  tubes,  and 
the  following  discussion  is  based  largely  on  his  publications 
in  regard  to  this  subject.  We  observed  in  paragraph  375 
that  a  great  many  of  the  electrons  radiated  from  the  fila- 
ment return  thereto  due  to  the  repelling  action  of  the  nega- 
tive space  charge.  If,  however,  the  plate  be  charged  with 
a  positive  potential,  its  charge,  being  opposite  to  that  of  the 
electrons,  will  serve  to  attract  them  to  it.  Consequently, 

239 


380]      ELEMENTS   OF  RADIOTELEGRAPHY. 

most  of  the  electrons  do  not  return  to  the  filament.  As  the 
positive  potential  of  the  plate  is  increased,  more  and  more 
electrons  are  attracted  toward  it.  We  have  previously 
observed  that  the  flow  of  a  stream  of  ions  between  two 
electrodes — in  this  case,  the  filament  and  the  plate — con- 
stitutes the  passage  of  a  current  of  electricity  between  the 
two  points.  In  the  electron  tube,  the  equivalent  current 
of  the  ionic  stream  projected  against  the  plate  is  termed  the 
plate  current  (or  the  wing  current).  Consequently,  with 
increased  plate  potential,  there  is  increased  plate  current. 

380.  The  relation  between  plate  potential  and  plate  cur- 
rent however,  is  not  that  of  Ohm's  Law.  If  it  were,  for 
every  increase  in  potential,  there  would  be  a  corresponding 
increase  in  current.  Instead,  the  following  action  takes 
place.  At  moderate  potentials  of  the  B  battery — which  sup- 
plies the  plate  potential — only  a  few  of  the  electrons  radi- 
ated reach  the  plate,  the  rest  are  returned  to  the  filament 
under  the  influence  of  the  space  charge.  As  the  plate  po- 
tential is  increased,  for  a  fixed  value  of  filament  current — 
and  hence  constant  electron  radiation — the  number  of 
electrons  reaching  the  plate  in  any  instant  of  time  will 
increase  up  to  that  amount  where  all  of  the  electrons  radi- 
ated are  attracted  to  the  plate.  The  space  charge  is  now 
completely  neutralized.  Further  increase  in  potential  can- 
not result  in  any  increase  in  the  plate  current.  Similarly, 
for  a  fixed  value  of  plate  potential,  an  increase  in  the  fila- 
ment current,  supplied  by  the  A  battery — so-called — will 
result  in  increased  electron  radiation  due  to  the  greater 
heat  generated.  A  greater  plate  current  results  from  this 
increased  ionic  radiation,  but  this  has  a  limiting  value  on 
account  of  the  greater  negative  space  charge  produced  by 
the  increasing  electron  emission.  As  the  A  battery  cur- 
rent is  increased,  the  space  charge  will  rise  to  a  value  equal 

240 


ELECTRON  TUBE   DETECTORS. 


[381 


to  that  of  the  plate  charge,  but  opposite  in  polarity.  The 
two  charges  thus  neutralize  each  other,  and  no  further 
attraction  can  be  exerted  by  the  plate,  so  that  the  addi- 
tional ions  radiated  are  returned  to  the  filament.  Thus, 
whether  we  leave  the  plate  potential  constant  and  vary  the 
degree  of  ionic  radiation,  or  leave  the  radiation  constant  and 
vary  the  plate  potential,  in  either  case  we  shall  find  that 
the  plate  current  will  rise  only  to  a  certain  value,  beyond 
which,  for  any  increase  in  either  the  radiation  or  attracting 
force,  it  cannot  grow  greater.  The  wing  current  at  this 
point  is  said  to  be  saturated.  (See  paragraph  270.) 

381.  For  a  given  plate  potential,  we  have  seen  that  the 
only  limiting  factor  to  the  plate  current  is  the  negative 
space  charge.  By  introducing  a  third  member  within  the 
tube,  termed  the  grid.  Lee  de  Forest,  an  American  inventor, 


Plate 
Current 


Grid  Potential 
Fig.  119.     Characteristic  of  Three-element  Tube. 

has  designed  a  tube  in  which  the  space  charge  can  be  con- 
trolled, thus  affecting  the  flow  of  plate  current.  (His  de- 
vice is  called  the  audion.)  When  the  grid  is  positively 
charged,  the  effect  of  the  negative  space  charge  will  be 
neutralized  and  a  greater  plate  current  will  flow.  On  the 
other  hand,  if  it  be  negatively  charged,  the  space  charge 
will  be  assisted,  resulting  in  driving  an  increased  number 

241 


382]      ELEMENTS   OF  RADIOTELEGRAPHY. 

of  electrons  back  to  the  filament,  with  consequent  dimi- 
nution of  plate  current.  The  relation  between  grid  poten- 
tial and  plate  current  is  shown  in  Fig.  119.  It  will  be  ob- 
served that  at  the  position  A  on  the  plate  current  curve, 
the  grid  is  at  zero  potential  with  respect  to  the  filament 
(negative  terminal).  For  a  positive  charge  of  the  grid 
with  respect  to  the  filament,  the  plate  current  is  increased 
until  the  point  B  is  reached,  when  the  space  charge  is 
completely  neutralized,  and  for  the  particular  degree  of 
filament  temperature  and  plate  potential  in  use,  no  more 
electrons  can  be  attracted  to  the  plate.  That  portion  of 
the  curve  past  B  is  the  saturation  current.  A  negative 
charge  on  the  grid  will  reinforce  the  space  charge,  reduc- 
ing the  plate  current  to  values  on  the  lower  portion  of  the 
curve. 

382.  With  the  grid  at  the  same  potential  as  the  negative 
side  of  the  filament,  the  plate  current  will  be  at  the  value 
A  of  Fig.  119.  This  is  of  course  a  direct  current,  and  plot- 
ted against  time,  as  shown  in  Fig.  120,  assumes  the  flat 


T 
Fig.  120. 

curve  represented  by  the  light  line.  If  an  alternating 
potential  be  now  impressed  upon  the  grid,  such  as  that  of 
an  incoming  oscillation,  a  fluctuation  in  the  wing  current 
will  occur,  of  the  same  frequency,  and  in  phase  with,  this 
periodic  E.M.F.  This  variation  in  the  plate  current  is 
shown  in  the  heavy  line  in  Fig.  120.  It  is  immaterial 
whether  the  impressed  E.M.F.  be  of  audio  or  ra.$io  fre- 

242 


ELECTRON   TUBE   DETECTORS. 


[383 


quency.  A  slight  alternating  potential  applied  to  the  grid, 
will  cause  a  very  large  fluctuation  in  the  wing  current,  as 
may  be  seen  from  Fig.  119.  A  very  slight  increase  in  the 
grid  potential  from  zero  to  a  value  indicated  by  D  will  cause 
the  wing  current  to  rise  from  A  to  B.  The  sensitiveness 
of  the  audion  as  a  relay  or  amplifier  is  thus  a  function  of  the 
steepness  of  this  curve,  for  the  steeper  the  curve,  the  greater 
will  be  the  wing  current  fluctuation  for  a  given  change  in 
grid  potential.  This  non-linear  characteristic  of  the  wing 


Fig.  121.     Electron  Tube  Detector. 

current  curve  of  the  audion,  i.e. — the  fact  that  it  does  not 
follow  Ohm's  Law  (see  paragraph  254) — is  responsible  for 
its  use  as  an  amplifier  in  both  transcontinental  telephony 
and  radiotelegraphy. 

383.  As  a  detector  of  spark  signals,  the  electron  tube 
makes  use  of  both  its  rectifying  and  amplifying  properties. 
19  243 


384]       ELEMENTS   OF  RADIOTELEGRAPHY. 

A  diagram  of  connections  is  shown  in  Fig.  121.  The  A 
battery  furnishes  current  for  heating  the  filament  F  and 
the  B  battery  supplies  the  required  positive  charge  on  the 
plate  P.  Incoming  oscillations,  when  properly  tuned  in  the 
antenna  and  secondary  circuits,  impress  a  radio  frequency 
potential  across  the  secondary  condenser  VC.  When  that 
side  of  the  stopping  condenser  SC,  which  is  connected  to 
the  grid,  is  positively  charged  by  this  radio  frequency  po- 
tential, a  large  current  passes  from  the  grid  to  the  fila- 
ment. (See  paragraph  377.)  When  it  is  negatively 
charged,  however,  the  rectifying  property  of  the  tube  will 
not  permit  a  current  to  flow  from  the  filament  to  the  grid. 
Consequently,  for  each  wave  in  an  incoming  train,  a  nega- 
tive charge  is  given  to  the  condenser  and  the  grid.  For 
each  train,  there  is  accumulated  on  the  grid  condenser  the 
summation  of  these  negative  charges.  This  large  negative 
charge  on  the  grid  causes  a  reduction  in  the  plate  current, 
according  to  Fig.  119. 

384.  The  nature  of  the  various  currents  and  potentials  is 
shown  in  Fig.  122.     It  should  be  borne  in  mind  that  the 
telephone  current  which  is  triggered  by  the  potential  on 
the  stopping  condenser  and  grid  is  of  audio  frequency.     The 
radio  frequency  plate  current  is  triggered  thru  the  telephone 
receivers  by  the  radio  frequency  potential  on  the  grid.     In 
the  simple  detector  connection  for  the  electron  valve  shown 
in  Fig.  121,  no  use  can  be  made  of  this  radio  frequency  cur- 
rent thru  the  telephone  receivers,  because  it  is  above  the 
limit  of  audibility  and  the  high  impedance  of  the  receivers 
reduces  its  value. 

385.  In  a  system  devised  by  Armstrong,  this  radio  fre- 
quency current  in  the  telephone  circuit  is  used  to  increase 
the  potential  in  the  grid  circuit,  thus  securing  an  increased 
amplification.     The  diagram  is  shown  in  Fig.  123.     Inserted 

244 


ELECTRON   TUBE   DETECTORS.  [385 


Radio  Frequency  Potential 


Plafe  Current 


Ro+entiol  on  Condenser 


Telephone  Current 


Fig.  122. 
245 


385]       ELEMENTS   OF  RADIOTELEGRAPHY. 

in  the  radio  frequency  or  secondary  circuit  is  the  secondary 
of  a  transformer  T,  the  primary  of  which  is  connected  in  the 
telephone  circuit.  The  variable  condenser  TC  across  the 
telephone  receivers  provides  a  low  impedance  path  for  the 
radio  frequency  plate  current.  This  radio  frequency  plate 
current,  which  is  supplied  by  the  B  battery  and  is  triggered 
by  the  radio  frequency  on  the  grid,  passes  thru  the  plate 
circuit  5,  7"C,  P,  F  and  the  primary  of  T.  It  is  considerably 


Fig.  123. 

larger  than  the  grid  current,  as  we  have  observed  from  a  con- 
sideration of  the  curve  in  Fig.  119.  With  the  windings  of  the 
primary  and  secondary  of  the  transformer  T  in  the  proper 
direction,  the  mutual  induction  between  the  two  windings 
will  transfer  some  energy  of  the  plate  current  into  the  sec- 
ondary circuit,  so  as  to  reinforce  the  original,  small,  grid 
potential.  The  additional  potential  now  impressed  upon  the 
grid  will  cause  an  even  greater  plate  current  to  flow,  and  the 

246 


ELECTRON  TUBE  AMPLIFIERS. 


[387 


energy  of  the  latter  is  fed  back  into  the  secondary  circuit  to 
further  reinforce  the  grid  potential.  This  regenerative  action 
thus  brings  about  a  tremendous  amplification  which  is  lim- 
ited only  by  the  constants  of  the  various  circuits  and  the 
tube.  The  final  potential  on  the  condenser  SC,  the  fourth 
curve  of  Fig.  122,  is  thus  very  much  larger  than  without  the 
regenerative  action,  and  the  telephone  current  is  accordingly 
greatly  increased. 

386.  Since  the  intensity  of  signals  depends  almost  wholly 
upon  the  value  of  the  grid  potential,  it  is  readily  seen  why 
tuned  coupled  circuits  must  be  used  in  order  that  a  large 
resonant  potential  may  be  built  up  in  the  secondary  circuit 
or  the  grid  circuit  KC,  Fy  G  and  SC. 

LVIII. 

ELECTRON  TUBE   AMPLIFIERS. 

387.  Combinations  of  more  than  one  tube  may  be  used 
to  secure  the  benefits  of  their  amplifying  properties.  *  It  is 


Fig.  124.     Detector  and  Amplifier  Circuit. 

obvious  that  by  applying  the  terminals  of  any  feeble  pul- 
sating current  circuit  to  the  grid  and  filament  respectively, 
an  amplification  of  this  current  may  be  obtained.  (This  is, 

247 


388]      ELEMENTS   OF  RADIOTELEGRAPHY. 

of  course,  utilization  of  the  non-linear  plate  current  curve.) 
The  simple  detector  connection  is  used  for  the  amplifying 
circuit  as  shown  in  Fig.  124.  It  will  be  noted  that  an  iron 
cored  transformer  is  used  to  transfer  energy  from  the  tele- 
phone circuit  of  the  first  audion  to  the  grid  circuit  of  the 
second.  There  is  a  very  great  number  of  connections 
which  may  be  used  for  combined  detector  and  amplifier 
combinations.  In  general,  they  provide  means  for  utilizing 
the  telephone  current  or  the  plate  current  to  trigger  local 
current  in  an  additional  audion.  Two  or  more  electron 
tubes  in  cascade  may  be  used  as  amplifiers,  but  as  more 
tubes  are  added,  their  operation  becomes  so  critical  that, 
as  a  rule,  it  is  not  customary  to  use  more  than  three  tubes. 
In  Fig.  124,  the  first  audion  is  used  as  a  combined  detector 
and  amplifier,  and  the  next  one  as  an  amplifier.  Additional 
amplifiers  are  added  by  inserting  the  primary  of  an  addi- 
tional iron  cored  transformer  in  the  place  of  the  telephone 
receivers,  and  continuing  the  same  scheme  of  connections. 

388.  The  amplifiers  may  be  so  connected  as  to  increase 
the  radio  current  (plate  current)  or  the  audio  current  (tele- 
phone current)  or  both.     Such  connections  are  quite  intri- 
cate, however,  and  the  reader  is  referred  to  the  appended 
bibliography  for  a  more  complete  discussion  of  this  subject. 

LIX. 
THE  HETERODYNE. 

389.  We  observed  in  paragraph  74  that  when  two  alter- 
nating currents  of  different  frequency  are  flowing  in  the 
same  circuit,  beats  are  formed.    The  frequency  of  the  beats 
is  equal  to  the  difference  between  the  frequencies  of  the 
two  oscillating  currents.     That  is  to  say,  if  one  current  be 
of  a  frequency  of  500,000  per  second  and  the  other  501,000 
per  second,  the  frequency  of  the  beats  will  be  1,000. 

248 


THE  HETERODYNE. 


[391 


390.  From  the  above,  it  will  be  seen  that  two  alternating 
currents  of  radio  frequency — which  are  inaudible — may  be 
superimposed,  one  on  the  other,  and  the  resultant  current 
rendered  audible.  Fessenden  adapted  this  principle  to  the 
reception  of  undamped  signals.  We  have  seen  that  the 
incoming  signals  from  an  undamped  wave  transmitter  such 


Fig.  125.     Heterodyne  Receiver. 

as  the  Poulsen  arc,  or  one  of  the  various  radio  frequency 
generators,  are  inaudible  unless  broken  up  into  separate 
wave  trains.  If  we  provide  some  local  source  of  radio 
frequency  current,  however,  which  we  can  superimpose 
on  the  received  current,  we  shall  be  able  to  produce 
audio  beats,  providing  the  two  currents  have  the  proper 
difference  in  frequency.  This  type  of  reception  is  termed 
the  heterodyne.  In  its  original  form,  it  comprised  a  circuit 
similar  to  that  shown  in  Fig.  125. 

391.  This  figure  shows  a  combination  of  a  small  Poulsen 
arc  generator  and  a  crystal  detector  receiver.  The  arc 
circuit  may  be  tuned  to  any  desired  frequency,  its  oscil- 
lations being  induced  into  the  secondary  circuit  of  the  re- 
ceiver by  means  of  the  two  couplers  M2  and  Mi.  The  os- 

249 


392]      ELEMENTS  OF  RADIOTELEGRAPHY. 

dilating  current  from  the  incoming  waves  is  also  induced 
into  this  circuit,  where  beats  are  formed  similar  to  the  wave 
trains  of  a  damped  wave  transmitter.  It  will  be  seen  that 
as  L  and  C  of  the  arc  circuit  are  varied,  the  frequency  of  the 
local  oscillations  may  be  detuned  any  desired  amount  from 
that  of  the  incoming  signals.  The  greater  the  difference 
in  frequency  between  the  two  alternating  currents,  the 
higher  will  be  the  frequency  of  the  beats  and  the  higher  the 
pitch  of  the  telephone  note.  The  receiving  operator  has 
thus  complete  control  over  the  note  of  signals,  which  is  not 
possible  with  the  average  receiver.  Further,  the  tone  is  of 
pure,  musical  quality  and  may  be  easily  distinguished  from 
atmospheric  disturbances.  A  certain  amplification  also 
takes  place  since  the  amplitude  of  the  beats  is  the  resultant 
of  that  of  the  received  and  local  currents. 

392.  This  system  of  detection  does  not  lend  itself  favor- 
ably to  spark  signal  reception,  since  there  will  be  superim- 
posed an  undamped  wave — the  arc  oscillations — on  the 
damped  current  of  the  received  oscillations.     Consequently, 
while  there  is  considerable  amplification,  the  note  of  the 
signals  is  rough  and  ragged.    The  higher  the  spark  fre- 
quency, however,  the  smoother  the  signals  become,  but  at 
no  time  is  the  tone  as  pleasing  as  when  receiving  signals 
from  an  undamped  wave  transmitter. 

393.  While  the  early  heterodyne  was  perfect  from  a  the- 
oretical standpoint,  it  was  found  very  difficult  to  secure  an 
arc  which  would  burn  sufficiently  smooth  and  quiet  to 
secure  perfect  beats.     As  a  result,  this  system  did  not  come 
into  very  wide  adoption  until  the  discovery  was  made  that 
oscillations  could  be  set  up  by  an  electron  tube  which  could 
be  tuned  and  superimposed  on  incoming  oscillations  in 
exactly  the  same  fashion  as  the  Fessenden  heterodyne. 

250 


AUDION  BEAT  RECEIVER.  [394 

LX. 
AUDION  BEAT  RECEIVER.1 

394.  In  paragraph  385,  we  observed  that  the  regenerative 
action  of  an  electron  tube  served  to  transfer  energy  back 
from  the  plate  circuit  to  the  grid  circuit.  If  the  coupling 
between  the  primary  and  secondary  of  the  transformer  T 
of  Fig.  123  be  made  sufficiently  close,  the  potential  induced 
in  the  grid  from  the  wing  circuit  is  so  great  as  to  maintain 
the  former  in  a  continuous  state  of  oscillation.  In  order 
that  the  oscillation  may  be  started,  however,  it  is  necessary 
to  produce  a  change  in  the  grid  potential  which  will  cause 
a  fluctuation  of  the  plate  current.  This  disturbance  in  the 
plate  circuit  feeds  back  a  pulsating  E.M.F.  into  the  grid 
circuit  which  maintains  its  oscillations.  It  is  sufficient  to 
lightly  tap  the  lead  to  the  grid  with  the  finger  so  as  to  change 
its  potential  to  start  the  complete  system  into  oscillation. 
As  a  matter  of  fact,  in  starting  the  current  thru  the  fila- 

1  Adaption  of  the  generation  of  undamped  oscillations  by  the  elec- 
tron tube  to  transmission  of  radio  signals  is  now  being  made  by  several 
American  and  European  radio  concerns.  In  this  country,  the  General 
Electric  Company  and  the  Western  Electric  Company  have  put  on  the 
market,  under  the  various  trade  names  listed  in  paragraph  373,  an 
electron  tube  of  large  size  which  is  suitable  for  the  production  of  un- 
damped oscillations.  The  system  of  connections  is  substantially  that 
shown  in  Fig.  123,  with  the  exception  that  the  telephone  receivers,  of 
course,  are  omitted.. 

As  a  generator  of  undamped  oscillations,  the  electron  tube  may  be 
employed  for  telegraphic  or  telephonic  purposes.  In  the  former  case, 
a  suitable  key  is  inserted  in  the  circuit,  and  if  reception  is  desired  by 
the  crystal  detector  method,  the  undamped  oscillations  are  modulated 
by  some  local  source  of  audio  frequency,  such  as  a  buzzer.  When 
radio  telephony  is  to  be  carried  on  by  this  method,  a  telephone  trans- 
mitter is  inserted  in  the  ground  lead. 

Such  transmitters  have  proved  highly  successful,  especially  where 
small  power  is  to  be  used,  as  on  aircraft. 

251 


395]       ELEMENTS   OF  RADIOTELEGRAPHY. 

ment  from  the  A  battery,  the  transient  ionic  current  and 
potential  fluctuation  are  sufficient  to  create  enough  of  a 
disturbance  between  the  plate  and  grid  circuits  to  set  up 
oscillations.  The  frequency  of  these  oscillations  is  largely 
determined  by  the  constants  of  the  grid  circuit. 

395.  In  the  reception  of  undamped  signals  by  the  com- 
bined audion-heterodyne  method,  the  local  oscillations  and 
the  incoming  oscillations  are  sufficiently  detuned  by  care- 
ful adjustment  of  the  capacities  VC,  SC,  and  TC  of  Fig.  123 
as  to  produce  beats  of  any  desired  frequency.    The  detec- 
tion action  is  then  the  same  as  discussed  in  the  earlier  part 
of  this  chapter  for  the  reception  of  damped  wave  signals. 

396.  While  the  possibilities  of  the  electron  tube  have 
been  suggested  rather  than  discussed  in  the  above,  due  to 
the  elementary  nature  of  this  text,  it  must  be  apparent  that 
the  opportunities  for  combined  heterodyne,  radio,  and  audio 
amplification  are  such  as  to  make  the  tube  an  extremely 
sensitive  piece  of  apparatus  for  continuous  wave  reception. 
Great  credit  is  due  Armstrong,  Austin,  de  Forest,  Moor- 
head  and  the  other  workers  in  this  line  for  their  develop- 
ment  of    this    most    interesting   device.     Long   distance 
transmission  by  radio,  to  which  accomplishment  we  are 
gradually  becoming  accustomed,  is  due  as  much  to  the 
development  of  the  modern  super-sensitive  receiver  as  it 
is  to  the  production  of  the  high  power  transmitter. 

LXI. 
MODERN  ELECTRON  TUBES. 

397.  The  de  Forest  audion  differs  from  most  of  the  other 
electron  tubes  on  the  market  in  that  traces  of  gas  are  left 
within  the  tube.    That  is  to  say,  the  audion  is  not  com- 
pletely exhausted  in  the  process  of  manufacture.     Since 

252 


PLATE   XXXII. 

Moorhead  Electron  Tube,  Type  B.     Suitable  for 
Transmitting  and  Receiving. 


PLATE  XXXIII. 

Moorhead  Electron  Relay  Suitable  for  Detector, 
Amplifier  and  Oscillator. 


MODERN  ELECTRON   TUBES.  [398 

there  are  left  undissociated  molecules  of  air  within  the 
tube  by  this  method,  the  electron  emission  produces  an 
ionization  of  the  tube  due  to  the  liberation  of  positive  and 
negative  ions  from  the  impact  of  collision.  (See  Section 
XIV.)  As  the  degree  of  vacuum  is  made  higher  and  higher 
in  the  process  of  exhaustion,  the  Edison  effect,  or  the  de- 
gree of  pure  electron  discharge  from  the  filament,  increases, 
since  it  is  easier  for  the  negative  corpuscles  to  detach  them- 
selves from  the  filament  with  the  pressure  of  the  surround- 
ing gas  removed.  As  the  tube  is  evacuated,  however,  the 
number  of  undissociated  molecules  is  diminished.  The 
resultant  ionization  depends  upon  both  of  these  factors, 
consequently  the  extent  of  ionization  will  vary  widely  for 
slightly  different  pressures.  Since  the  shape  of  the  curve 
of  Fig.  119,  in  a  de  Forest  audion,  is  a  function  of  the  ex- 
tent of  ionization,  the  difficulty  in  exactly  securing  the  cor- 
rect pressure  for  optimum  ionization  results  in  the  produc- 
tion of  audions  of  widely  differing  degrees  of  sensitivity. 

398.  Several  tubes  are  now  on  the  market  in  which  only 
the  ionic  discharge  from  the  filament  is  used.  In  the  man- 
ufacture of  these  tubes,  all  traces  of  gas  are  completely 
removed,  consequently  tubes  of  consistent  performance 
may  be  turned  out.  Electron  tubes  of  the  Western  Elec- 
tric Company,  General  Electric  Company  (the  so-called 
kenotron  and  pliotron)  and  of  a  San  Francisco  manufac- 
turer, O.  B.  Moorhead,  are  so  produced.  The  latter 
makes  use  of  the  electro-chemical  relation  existing  be- 
tween certain  metals  in  the  construction  of  the  filament, 
wing  and  grid.  These  tubes  are  characterized  by  great 
sensitivity  and  uniform  performance  but,  due  to  the  di- 
minished conductivity  caused  by  the  elimination  of  gas  ion- 
ization, require  greater  plate  potential,  as  supplied  by  the 
B  battery,  than  does  the  de  Forest  audion. 
20  253 


399]      ELEMENTS   OF  RADIOTELEGRAPHY. 

LXII. 
MAGNETIC   CONTROL. 

399.  In  paragraph  382,  we  observed  that  as  a  simple  de- 
tector or  amplifier,  i.e.,  non-regenerative,  the  sensitivity  of 
the  electron  tube  is  wholly  a  function  of  the  steepness  of 
the  curve  of  Fig.  119.     It  will  be  found  that  if  a  magnet  be 
brought  within  a  short  distance  of  a  tube  when  so  used,  its 
sensitivity  can  be  enormously  increased  by  certain  critical 
adjustments  of  the  position  of  the  magnet.     In  1914,  the 
author  published  a  discussion  of  this  subject,  illustrated 
with  plate  current  curves  similar  to  that  in  Fig.  119,  show- 
ing that  the  increase  in  signal  strength  due  to  the  magnet 
was  caused  by  increased  steepness  of  this  curve,  and 
ascribing  this  result  to  deflection  of  the  ions  such  as  to 
bring  about  a  critical  balance  between  the  electrostatic 
influence  of  the  grid,  filament,  and  plate  potentials  and  the 
magnetic  field.     Thus,  a  slight  change  in  the  grid  potentials 
from  the  incoming  oscillations  would  exert  the  double  ac- 
tion of  affecting  the  space  charge  as  well  as  restoring  the 
deflected  ions  to  their  normal  course,  thereby  causing  an 
even  greater  change  in  the  telephone  current.     (See  para- 
graph 261 -a.) 

400.  The  effect  of  the  magnetic  field  is  not  so  noticeable 
when  the  regenerative  and  beat  actions  of  the  tube  are 
brought  into  play,  probably  due  to  the  fact  that  the  ampli- 
fication obtainable  by  the  magnet  is  but  a  small  proportion 
of  the  enormous  amplification  secured  by  the  other  methods. 

LXIII. 

CONCLUSION. 

401.  The  practical  applications  of  radio  telegraphy,  and 
its  companion  communication — radiotelephony,  have  never 

254 


CONCLUSION.  [401 

been  so  pronounced  as  at  present,  during  the  war.  Fire 
control  ashore  and  at  sea  is  now  directed  by  observers  in 
radio  equipped  airplanes,  while  the  more  common  radio 
communication  in  the  field,  between  units  of  the  fleet  and 
with  land  are  familiar  examples  of  its  military  adoption. 
The  use  of  various  forms  of  radio  direction  finders  or  com- 
passes has  made  it  possible  to  guide  air  craft  and  to  locate 
enemy  radio  stations — ashore  and  afloat.  Distances  of 
12,500  miles — half  the  circumference  of  the  earth — by  the 
radio  telegraph,  and  4,700  miles  by  the  radio  telephone, 
have  already  been  covered.  While  it  is  highly  improbable 
that  radiotelegraphy  and  radiotelephony  will  ever  com- 
pletely supersede  the  wire  telegraph  and  telephone,  their 
practicability,  efficiency,  and  general  worth  are  certainly 
established  facts. 


255 


APPENDIX. 

In  the  preparation  of  the  lectures  of  which  this  text  is  a 
reprint,  the  author  had  occasion  to  make  frequent  use  of 
the  following  publications,  to  whose  writers  he  gratefully 
acknowledges  his  indebtedness. 

BIBLIOGRAPHY. 

Eccles,  W.  H.     "Wireless  Telegraphy  and  Telephony." 

Fleming,   J.  A.     "  The  Principles  of  Electric   Wave  Telegraphy  and 

Telephony." 

Kimball,  A.  L.     "A  College  Text-book  of  Physics." 
McClung,  R.  K.     "Conduction  of  Electricity  thru  Gases  and  Radio- 

Activity." 

Pierce,  G.  W.     "  Principles  of  Wireless  Telegraphy." 
Sheldon-Hausmann.     "  Dynamo  Electric  Machinery."     (Parts  1  and 

2.) 

Steinmetz,  C.  P.     "Alternating  Current  Phenomena." 
Zenneck,  J.     "  Wireless  Telegraphy."     (Translated  by  Seelig.) 

PUBLICATIONS   OF  THE  BUREAU   OF   STANDARDS. 

Bulletin,  Vol.  3,  No.  2.  Austin,  L.  W.  "  The  Production  of  High 
Frequency  Oscillations  from  the  Electric  Arc." 

Bulletin,  Vol.  6,  No.  4.  Austin,  L.  W.  "  The  Comparative  Sensitive- 
ness of  Some  Common  Detectors  of  Electrical  Oscillations." 

Bulletin,  Vol.  7,  No.  3.  Austin,  L.  W.  "  Some  Quantitative  Experi- 
ments in  Long  Distance  Radiotelegraphy." 

Bulletin,  Vol.  9.     Austin,  L.  W.     "  Antenna  Resistance." 

Scientific  Paper  No.  158.  Austin,  L.  W.  "  Some  Experiments  with 
Coupled  High  Frequency  Circuits." 

Scientific  Paper  No.  226.  Austin,  L.  W.  "  Quantitative  Experiments 
in  Radiotelegraphic  Transmission." 

Scientific  Paper  No.  235.  Kolster,  F.  A.  "  A  Direct-Reading  Instru- 
ment for  Measuring  the  Logarithmic  Decrement  and  Wave  Length 
of  Electro-Magnetic  Waves." 

256 


BIBLIOGRAPHY. 

Scientific  Paper  No.  257.  Austin,  L.  W.  "  Note  on  the  Resistance  of 
Radiotelegraphic  Antennas.11 

Scientific  Paper  No.  269.  Miller,  J.  M.  "  Effect  of  Imperfect  Dielec- 
trics in  the  Field  of  a  Radiotelegraphic  Antenna." 

Circular  No.  74.     "  Radio  Instruments  and  Measurements." 

"PROCEEDINGS"    OF  THE  INSTITUTE    OF  RADIO 
ENGINEERS. 

Institute  of  Radio  Engineers.  "  Report  of  the  Committee  on  Standard- 
ization for  1915." 

Vol.  1,  No.  1.  De  Forest,  Lee.  "Recent  Developments  in  the  Work 
of  the  Federal  Telegraph  Company." 

Vol.  1,  No.  2.  Kolster,  F.  A.  "  The  Effects  of  Distributed  Capacity  of 
Coils  used  in  Radio  Telegraph  Circuits." 

Vol.  1,  No.  3.  Seelig,  A.  E.  "  The  Sayville  Station  of  the  Atlantic 
Communication  Company." 

Vol.  1,  No.  3.  Kennelly,  A.  E.  "  The  Daylight  Effect  in  Radio  Teleg- 
raphy." 

Vol.  1,  No.  4.    Rein,  Hans.     "  The  Multitone  System." 

Vol.  1,  No.  4.  Weagant,  R.  A.  "  Some  Recent  Radio  Sets  of  the  Mar- 
coni Wireless  Telegraph  Company  of  America." 

Vol.  2,  No.  1.  De  Forest,  Lee.  "  The  Audion — Detector  and  Ampli- 
fier." 

Vol.  2,  No.  4.  Eastham,  M.  "  The  '  Hytone '  Radio  Telegraph  Trans- 
mitter." 

Vol.  3,  No.  1.  Kolster,  F.  A.  "A  Direct-Reading  Decremeter  and 
Wave  Meter." 

Vol.  3,  No.  2.  Hallborg,  H.  E.  "  Resonance  Phenomena  in  the  Low 
Frequency  Circuit  of  Radio  Transmitters." 

Vol.  3,  No.  3.  Armstrong,  E.  H.  "  Some  Recent  Developments  in  the 
Audion  Receiver." 

Vol.  3,  No.  3.  Hogan,  J.  L.  "  Developments  of  the  Heterodyne  Re- 
ceiver." 

Vol.  3,  No.  3.  Langmuir,  I.  "  The  Pure  Electron  Discharge,  and  Its 
Application  in  Radio  Telegraphy  and  Telephony." 

Vol.  4,  No.  3.     Stone,  E.  W.     "  An  Impulse  Excitation  Transmitter." 

Vol.  4,  No.  3.  Austin,  L.  W.  "  Experiments  at  the  U.  S.  Naval  Radio 
Station,  Darien,  Canal  Zone." 

Vol.  4,  No.  3.  Lowenstein,  F.  "  The  Mechanism  of  Radiation  and 
Propagation  in  Radio  Communication." 

257 


ELEMENTS   OF  RADIOTELEGRAPHY. 

Vol.  4.  No.  4.     Shoemaker,  H.     "  Recent  Standard  Radio  Sets." 

Vol.  4,  No.  5.     Hogan,  J.  L.     "  Physical  Aspects  of  Radio  Telegraphy." 

Vol.  4,  No.  5.  Fuller,  F.  L.  "  A  Few  Experiments  with  Ground  An- 
tennas." 

Vol.  4,  No.  6.     Marchant,  E.  W.     "  The  Heaviside  Layer." 

Vol.  5,  No.  1.  Marriott,  R.  H.  "Engineering  Precautions  in  Radio 
Installations." 

Vol.  5,  No.  2.  Stone,  E.  W.  "  Some  Additional  Experiments  on  Im- 
pulse Excitation." 

Vol.  5,  No.  2.  Armstrong,  E.  H.  "  A  Study  of  Heterodyne  Ampli- 
fication by  the  Electron  Relay." 

Vol.  5,  No.  3.  Bouthillon,  Leon.  "  On  the  Use  of  Constant  Potential 
Generators  for  Charging  Radio  Telegraphic  Condensers  and  the 
New  Radio  Telegraphic  Installations  of  the  Postal  and  Telegraphic 
Department  of  France." 

Vol.  5,  No.  4.  Austin,  L.  W.  "  The  Measurement  of  Radiotelegraphic 
Signals  with  the  Oscillating  Audion." 

Vol.  5,  No.  4.    Pedersen,  P.  O.     "  On  the  Poulsen  Arc  and  Its  Theory." 

Vol.  5,  No.  5.  Earth,  Julian.  "  The  Effect  of  Commercial  Conditions 
on  Spark  Transmitter  Construction." 

Vol.  5,  No.  6.  Morecroft,  J.  H.  "Some  Experiments  with  Long 
Electrical  Conductors." 

Vol.  5,  No.  6.  Stone,  E.  W.  "Municipal  Regulations  for  Radio 
Stations." 

Vol.  5,  No.  6.  Moorhead,  O.  B.  "  The  Manufacture  of  Vacuum  De- 
tectors." 

Vol.  6,  No.  1.     Hull,  A.  W.     "  The  Dynatron." 

Vol.  6,  No.  1.  Taylor,  H.  O.  "  Telephone  Receivers  and  Radio  Teleg- 
raphy." 

Vol.  6,  No.  2.    Hazeltine,  E.  W.     "  Oscillating  Audion  Circuits." 


258 


INDEX. 


Numbers  refer  to  paragraphs. 


Absorption,  of  ions,  103. 

of  waves  in  transmission,  316. 
Aerials,  various  types  of,  287. 

See  Antennae. 
Air  blast,  for  spark  gaps,  101,  102, 

113,  123. 
Aircraft,      radio      communication 

with,  324. 

ALEXANDERSON,  E.  F.  W.,  247. 
Alternating  current,  theory  of,  31. 
Alternators,  radio  frequency,  for 

undamped  waves,  247. 
Ampere,  definition  of,  16. 
Antennae,  276. 

earthing    of— similar   image, 
286. 

for  aircraft,  325. 

harmonic  oscillation  of,  282. 

resistance,  305. 

theory  of,  276. 

towers  for,  294. 

types  of,  287. 
Arc  (Poulsen), 

adoption  of,  251,  275. 

deionization  of,  102,  106,  257, 
261,  264. 

harmonics  of,  259,  283^ 

theory  of,  252. 

time  periods  of,  258. 

transmitting  keys  for,  268. 
ARMSTRONG,  E.  H.  379,  396. 
See  Bibliography. 


Atmosphere,   effect   of,  on  wave 

propagation,  321. 
Audibility,  measurement  of,  369. 
Audion,  De  Forest,  373,  397. 

See  Electron  Tubes. 
AUSTIN,  L.  W.     See  Bibliogra- 
phy. 

on  antenna  resistance,  307. 

on  the  tikker,  353. 

on  electron  tubes,  396. 
Automatic  keys,  125. 

B 

BJERKNES,  V.,  formula  of  for 
logarithmic  decrement  measure- 
ment, 227. 

BLACK,  R.  B.    See  Foreword. 
Blowers,  air,  101,  102,  113,  123. 
Blow-out,  magnetic,  106,  261. 
BOUTHILLON,  LIEUT.  LEON, 

See  Bibliography. 
Break  key,  175. 

Brush  discharge,  of  condensers, 
145. 

C 

Capacity.     See  Condensers, 
definition  of,  23. 
distributed,  in  coil,  359. 
formulae  for,  24,  25. 
of  aircraft  antennae,  325. 
of  antennae,  287,  292. 
of  compressed  air  condensers, 
147. 


259 


ELEMENTS   OF  RADIOTELEGRAPHY. 


Capacity  of  condensers  in  series, 

25,  144. 
of  condensers  in  parallel,  25, 

144. 

of  counterpoise,  303. 
of  Navy  condensers,  149. 
of  plate  condensers,  144. 
specific  inductive,  24. 
value  of,  for  different  dielec- 
trics, 24. 
Carbon,  in  the  Poulsen  arc,  256, 

261,  262,  265. 

Carborundum  detector,  345. 
Charging  period  of  the  arc,  258. 
Choke  coils,  of  Poulsen  arc  trans- 
mitter, 185,  260. 
Chopper,  250. 
Coherer,  330,  332,  338. 
Coils.     See  Inductances. 
Compressed  air,  condensers,  147, 

207. 

in  spark  gaps,  105. 
Condensers.     See  Capacity, 
fixed,  361. 
protection  of,  148. 
receiving,  361. 
types  of,  143. 
variable,  362. 
Counterpoise,  298,  303. 
Coupled  circuits,  73. 
Coupling,  73,  237. 

capacity  of  static,  75. 
inductive,  75. 
of  receivers,  335,  337. 
of  tickler  coil,  394. 
Crystal  detectors,  344. 
Curvature  of  the  earth,  effect  on 
transmission,  313,  317. 


Damped  wave,  45. 


Damping.     See  Decrement, 
measurement  of,  223. 
of  condenser  discharge,  26, 45. 
of  waves,  45,  47,  49. 
Daylight,  effect  of,  on  transmis- 
sion, 320,  321. 

Decrement,  linear,  56  and  foot- 
note. 

logarithmic.     See  Damping, 
definition  of,  50. 
effect  of,  on  resonance, 

58,  59. 
formulae  for,  55,  56,  227, 

230,  231. 

measurement  of,  223. 
Decremeter,  222. 
Fleming,  234. 
Kolster,  233. 
Marconi,  234. 
DE  FOREST,  LEE,  381,  396. 

See  Bibliography, 
audion,  373,  397.     See  Elec- 
tron Tubes. 
Deionization  of  spark  gap,  98,  101. 

of  the  arc,  257,261. 
Detectors,  crystal,  344. 
electrolytic,  339. 
magnetic,  348,  349. 
tikker,  250,  350. 
valve,  378. 

Detuning,  in  decrement  measure- 
ment, 223. 
of    Telefunken   transmitters, 

116. 
Dielectric,  definition  of,  22. 

effect  on  capacity  of  conden- 
ser, 24. 
Diffusion  of  ions  in  arc,  261,  262. 

in  spark  gap,  102. 
Directive  antennae,  288,  289,  290, 
292,  293. 


260 


INDEX. 


Discharge  of  condenser,  26. 

Discharging  period  of  the  arc, 
258. 

Double  frequency  oscillations,  of 
coupled  tuned  circuit  transmit- 
ter, 109. 

E 

Earth.     See  Ground. 

Earth's  surface,  effect  of,  on  trans- 
mission, 313. 

ECCLES,  W.  H.  See  Bibliogra- 
phy- 

Eddy  currents  in  transformer 
core,  134. 

Edison  effect,  373. 

Electrolytic  detector,  339. 

Electron,  definition  of,  94. 

Energy,  transfer  of  between  coup- 
led circuits,  73,  76. 

Excitation,  impulse.  See  Im- 
pulse Excitation. 

Extinction  voltage  of  arc,  257,  258, 
261,  264,  265. 


Farad,  definition  of,  24. 
FESSENDEN,  R.  A.,  compressed 

air  condenser,  105,  147. 
electrolytic  detector,  339. 
heterodyne,  390. 
transmitter,  207. 

Field,  electric — of  spark  gap,  104. 
magnetic,  for  deionization  of 
spark  and   arc,    106,   257, 
261,  263. 

poles,  of  an  alternator,  10. 
FLEMING,  J.   A.     See   Bibliog- 
raphy. 

decremeter,  234. 
valve,  378. 
Frequency,  audio,  defined,  31. 


Frequency,  definition  of,  3,  31. 
of  an  alternator,  32. 
of  harmonics,  259,  283. 
of  oscillatory  circuits,  63. 
Frequency,  radio,  defined,  31. 
FULLER,  L.  F.    See  Bibliography. 
Fundamental  oscillation,  of  an  an- 
tenna, 282,  283. 
of  a  vibratory  body,  277. 

G 

Galvanometer,  for  detecting  small 

currents,  30,  376. 
Gap,  spark.     See  Spark  Gap. 
Generator,  for  A.  C.,  32. 

principle  of,  30. 

protection  of,  185,  260. 

similarity  of  motor  to,  183. 
GOLDSCHMIDT,  R.,  radio  fre- 
quency alternator,  247,  249. 
Ground,  antenna,  293. 

as  image  of  antenna,  286. 

connection,  298. 

currents,  300,  315. 

effect  on  transmission,  313. 

effect  on  waves  from  aircraft, 
326. 

loop  of  current  at,  281. 

of  aircraft,  325. 

of  vessels,  304. 

plate,  302. 

replacing  of,  by  counterpoise, 
303. 

resistance,  316. 

H 

HALLBORG,  H.  E.     See  Bibli- 
ography. 
Harmonic    oscillations,    antenna, 

282. 

arc,  259. 
vibratory  bodies,  277. 


261 


ELEMENTS  OF  RADIOTELEGRAPHY. 


Harp  antenna,  291. 

HAUSMANN,   E.     See    Bibliog- 
raphy. 

Heatwaves,  2,5,  7,  11. 

Henry,  definition  of,  28. 

HERTZ,  H.,  experiments  of,  69. 
waves,  313,  317,  325. 

Heterodyne,  389,  396. 

High  frequency  alternators,  247. 

HOGAN,  J.  L.     See  Bibliography. 

Hot  wire  ammeter,  167. 

HULL,  A.  W.     See  Bibliography. 

Hydrocarbon  gas  for  ionic  diffu- 
sion, 102,  261. 

Hydrogen,  see  above. 

Hysteresis,  in  transformer  core, 

135. 
of  magnetic  detector,  348. 


lonization,   magnetic    control  'of, 
106. 
261  (a),  399. 

of  the  air,  94,  321. 

of  the  arc,  256. 

of  spark  gaps,  95,  99. 

theory  of,  89. 
Iron,  magnetic  properties  of,  27. 

eddy  currents  in,  134. 

hysteresis  of,  135. 


KENNELLY,  A.  E.  See  Bibliog- 
raphy. 

KIMBALL,  A.  L.  See  Bibliog- 
raphy. 

KOLSTER,  F.  A.  See  Bibliog- 
raphy, 233,  294. 


Ignition  voltage  of  arc,  255,  257, 

258,  261,  264,  265. 
Image  theory  of  ground,  286. 
Impulse  excitation,  83,  105,  159, 

191,  195,  204,  208. 
Inductances,  27. 
Induction,  electro  magnetic,  30. 
Inductive  coupling,  75. 
Infra  red  waves,  7,  11. 
Interference  between  stations,  58, 

59. 
Intermediate   circuit  in   Marconi 

receiver,  357. 
Interrupter,    for   induction    coils, 

139. 
in  antenna  circuits  (chopper), 

250. 

in     receivers    (tikker),    250, 
350. 

lonization,  effect  of  atmospheric, 
on  transmission,  321. 


LANGMUIR,   I.     See   Bibliogra- 
phy. 

Leakage,  145. 

Length  of  spark  gaps,  effect  of, 
103,  113,  159,  201,  205. 

LEPEL,  E.  VON,  112,  208. 

Light  waves,  5,  6,  9. 

Linear  decrement,  56   and  foot- 
note. 

Location  of  radio  stations  by  di- 
rection finders,  401. 

LODGE,  OLIVER,  receiver,  332. 
transmitter,  81, 

Logarithmic      decrement.        See 
Decrement,  Logarithmic. 

Loop  antenna,  294. 

Loops,  of  potential  and  current, 

280. 
of  string  vibration,  277. 

LORENZ,  C.,  transmitter,  208. 


262 


INDEX. 


LOWENSTEIN,  F.     See  Bibliog-      MOORHEAD,  O.  B.     See  Bibli- 


raphy. 


M 


Magnetic  blow  out,  106,  261. 
Magnetic  detector,  348. 
Magnetic  field,  106,  257,  261,  263. 
MARCHANT,  E.  W.     See  Bibli- 
ography. 
MARCONI,     G.,    and     Marconi 

Wireless  Tel.  Co.'s, 
1896  transmitter  and  receiver, 

69,  330. 
1900  transmitter  and  receiver, 

108,  335. 

antennae  for  high  power  sta- 
tions of,  290. 
decremeter,  234. 
intermediate  receiver  circuit 

of,  357. 

magnetic  detector,  348. 
system,  188. 

variable  condensers,  362,  363. 
MARRIOTT,  R.  H.    See  Bibliog- 
raphy. 
MASSIE,   W.   W.,    sealed   point 

electrolytic  detector,  340. 
MAXWELL,  J.  C.,  theory  of  light, 

10. 

McCLUNG,  R.  K.     See  Bibliog- 
raphy. 
Mercury  arc  rectifier,  theory  of, 

195. 
use   of  in   Simpson 

transmitter,  200. 
Metal  filing  coherer,  330. 
Mica  condensers,  152,  192. 
Microfarad,  definition  of,  24. 
MILLER,  J.  M.     See  Bibliogra- 
phy, 
on  antenna  resistance,  307. 


ography. 

MORECROFT,  J.  H.  See  Bibli- 
ography. 

MOSCICKI,  J.,  condenser,  151. 

Mountains,  effect  of,  on  transmis- 
sion, 314. 

Multitone  transmitter,  208. 

N 

National  Electric  Signaling  Com- 
pany, 207. 

Navy,  TJ.  S.,  adoption  of  carbor- 
undum detector, 
345. 

Fessenden  transmit- 
ter, 234. 

Goldschmidt  alter- 
nator, 249. 
Kilbourne    &   Clark 

transmitter,  203. 
Poulsen  arc,  251, 

275. 

Telefunken      trans- 
mitter, 190. 
antennae,  290. 
audibility  measurements, 

370. 

decremeters,  249. 
radio  laboratory,  307. 
towers,  295. 
Nodes,  of  current  and  potential, 

280. 
of  vibratory  strings,  277. 


Ohm,  definition  of,  16. 

Oil,  for  reduction  of  brush  dis- 
charge, 145,  204. 
for  dielectric  of  condensers, 
146. 


263 


ELEMENTS   OF  RADIOTELEGRAPHY. 


Oil  keys,  123. 

Oscillations,  arc.     See  Arc  (Poul- 

sen). 

damping    of.       See     Decre- 
ment. 

forced,  61,  247. 
free,  61. 

harmonic.     See  Harmonics, 
of  antennae,  280. 
of  beat  (valve)  receivers,  394. 
of  condenser  circuits,  26,  45, 

61,  246. 

of  coupled  circuits,  73. 
of  LODGE  receiver,  333,  337. 

transmitter,  83. 
of  MARCONI  receivers,  330, 

335,  337. 

transmitters,  70,  109. 
of  pendulum,  26,  46,  61,  62, 

73. 
of     SIMPSON     transmitter, 

200. 
of    Telefunken    transmitter, 

192. 
of  tone  circuit  of  Multitone 

transmitter,  209. 
of  vibratory  strings,  277. 


Parallel,  condensers  in,  25,  144. 

Partial  spark  discharges,  209. 

PEDERSEN,  P.  O.     See  Bibliog- 
raphy, 
on  Poulsen  arc,  265. 

Pendulum,  oscillations  of,  26,  46, 
61,  62,  73,  100. 

Perikon  detector,  344. 

Period  of  condenser  circuit,  64. 

Phase,  definition  of,  40. 

PICKARD,  G.  W.,  on  crystal  de- 
tectors, 344. 


PIERCE,  G.  W.  See  Bibliograhpy. 

on  crystal  detectors,  344. 
Plates,  condenser,  144,  146,  204. 
or  disks  for  quenched  gaps, 
99,  103,  104,  113,  159,  160, 
201,  205. 

Polarization    of    electrolytic    de- 
tector, 339. 
Potential,  definition  of,  16. 

distribution   of,    of   antenna, 

280,  297. 

extinction,  of  arc,  257. 
ignition,  of  arc,  255. 
Potentiometer,  341. 
POULSEN,  V.     See  Arc   (Poul- 
sen). 

inventor  of  arc,  251. 
Power,  definition  of,  17. 

formula  for,  17. 

Propagation  of    waves  from  air- 
craft, 324. 

over  earth's  surface,  313. 
Pulsating  current,  140,  194,  195, 
334,  343,  346. 


Quenched  spark  gap,  88,  101,  113, 
114,  118,  158,  159,  160,  188,  190, 
204,  208,  236. 

Radiation    from,  inverted  L  an- 
tenna, 288. 

Lodge  transmitter,  82. 
Marconi  transmitters,  70, 

109,  188. 

Navy  antennae,  290. 
Poulsen  transmitter,  273. 
Simpson  transmitter,  200. 
Telefunken  transmitter, 

115. 

Thompson  transmitter, 
193. 


264 


INDEX. 


Radiation  from  umbrella  antenna,      Resistance,  definition  of,  15. 


292. 

of  harmonics,  259,  283,  295. 
heat  waves,  2. 
light  waves,  5. 
low  decrement  waves, 

59. 

Radium,  presence  of,  94,  99. 
Reactance,  33,  66. 

antenna,  283  and  footnote. 
Receiver,  harmonic  oscillation  of, 

371. 

Lodge,  332. 
Marconi,  330,  335. 
modern,  356. 
pioneer,  329. 
telephone,  365. 
Receiving  circuits,  329. 
condensers,  361. 
transformers,  358. 
Rectifier,  crystal,  344. 
electrolytic,  339. 
electron  tube,  377,  383. 
mercury  arc,  195. 
Reflection,  of  waves  on  strings, 

277. 

of  waves   by   upper    atmos- 
phere, 322. 
Refraction  of    propagated  waves, 

323. 

Reignition  of  arc,  255,  257. 
REIN,  H.     See  Bibliography. 
Relay,  Brown,  for  receiving  mag- 
nification, 368. 
key,  122. 
polarized,  368. 
Repulsion,  ionic,  of  electron  tube, 

375,  379. 
of  solution,  92. 

Resistance,  antenna,  71,  82,  305. 
arc,  255,  257. 


formulae  for,  20. 

ground,  298,  305,  310,  316. 

high  frequency,  164, 

Joulean,  305. 

Ohmic,  305. 

radiation,  56,  305. 

spark  gap,   71,  85,  98,  305, 

310,  316. 

Resonance,  39,  45,  66,  77,   110, 
116, 136,  244,  259,  283,  294,  332. 
335,  368. 
Rheostat    for    motor     generator 

speed  control,  182. 
Rotary  spark  gaps,  153,  155,  188, 

207,  212. 

RUTHERFORD,  E.,  magnetic  de- 
tector, 348. 


SCHELLER,  O.,  113. 

Seawater,   propagation  of  waves 

over,  316,  320. 

SEELIG,  A.  E.     See  Bibliography. 
Self  induction,  27,  76,  238. 
Series  connection  of  condensers, 

144,  172. 

SHELD  O  N,  S.    See  Bibliography. 
Ship  antennae,  288. 
SHOEMAKER,  H.     See  Bibliog- 
raphy. 

Silicon  detector,  344. 
Skin  effect,  164. 
Soil,  effect   of,   on  transmission, 

316. 
Spark  gap,  deionization  of,  98. 

Haller  Cunningham,  205. 
impulse,   105,   159,    160, 

208. 

Lodge,  82. 
Lorenz,  208. 


265 


ELEMENTS   OF  RADIOTELEGRAPHY. 


Spark  gap,  Marconi,  70,  108,  188. 
open,  154. 
quenched.  See  Quenched 

Spark. 

rotary,     155,    188,    207, 
212. 

Simpson,  200,  201. 
Telefunken,  114,  190. 
theory  of,  93. 
Thompson,  192,  201. 
types  of,  153. 

Spectrum  of  light  waves,  7. 
STEINMETZ,  C.  P.     See  Bibli- 
ography. 
STONE,    LIEUT.    E.    W.       See 

Bibliography, 
deionization    of    spark   gaps, 

105. 

impulse  gap,  159,  205. 
protection    of    low    potential 

circuits,  185. 

rotary  quenched  gap,  160. 
Sun,  effect  of,  on  wave  propaga- 
tion, 320,  321. 
Surface  waves,  69,  313. 

T 

Tapper  for  coherer,  330. 
TAYLOR,  H.  O.     See  Bibliogra- 
phy. 
Telefunken,  detuning  of  gap  and 

antenna  circuits,  116. 
polarized  relay,  368. 
quenched  spark  gap,  113,  190. 
sparkless  key,  124. 
system,  190. 
towers,  295. 
transmitter,  114. 
umbrella  antenna,  292. 
untuned  secondary  receiver, 
334. 


Telefunken  variometer,  166. 
Telephone,  radio,  401. 

receiver  current,  384,  399. 
resistance,  367. 
types  of,  366. 
use  of,   with   Lodge  re- 
ceiver, 333. 
Marconi      receiver, 

335. 

valve  receiver,  378. 
relay,  Brown,  368. 

Telefunken,  368. 

Thermocouple  with  antenna  am- 
meter, 169. 

THOMPSON,  R.  E.,  impulse  ex- 
citation transmitter,  191. 
Tickler  coil,  385. 
Tikker,  250,  350. 

Tone  circuit,  with  impulse  trans- 
mitter, 208. 

Transformer  core,  134.  . 
description  of,  128. 
eddy  currents  in  core  of,  134. 
hysteresis  of  core  of,  135. 
Marconi  receiver,  335,  337. 
potential  relation,  131. 
receiving,  358. 
resonant,  136. 
tickler,  385. 
types  of,  133. 

Trees,  effect  of,  on  antenna  resist- 
ance, 308. 
Tube,  electron,  Edison  effect  in, 

373. 
grid  current,  379. 

potential,  379. 
magnetic  control  of,  399. 
modern,  397. 
plate  current,  379. 
potential,  379. 
Tuning.     See  Resistance. 


266 


INDEX. 


Ultra-violet  light,  7,  84,  88,  94,  99. 

112. 

Umbrella  antenna,  292. 
Undamped  wave  transmitters,  arc, 

251. 
radio  frequency    alternators, 

246. 
Unilateral  conductivity  of  crystal 

detector,  344. 
electrolytic  detector,  339. 
electron  tube,  377,  383. 
mercury  arc,  195. 
Poulsen  arc,  259. 
Untuned  secondary  receiver,  334, 
337,  357. 

V 
Vacuum    detectors.      See    Tube, 

electron. 

Valve  detectors.     See  Tube,  elec- 
tron. 

Variometer,  Telefunken,  166. 
Vector  diagram  of  A.  C.,  37. 
Velocity  of  radio  waves,  10. 
Visible  spectrum,  7. 
Volt,  definition  of,  16. 


W 

Water,  effect  of,  on  ground  connec- 
tion, 299, 302, 

304. 
propagation  of  waves  over, 

316,  320. 
Watt,  definition  of,  17. 

meter,  Fig.  44. 

Wave  length,  formulae  for,  64. 
of  heat  waves,  3. 
light  waves,  5. 
radio  waves,  11. 
sound  waves,  4. 
meter,  description,  217. 

use  of,  236. 

WEAGANT,  R.  See  Bibliography. 
WIEN,  M.,  quenched  gap,  112. 


Z 

J.     See 


Bibliogra- 


ZENNECK, 

phy. 

deionization    of    spark    gap, 

100. 

ground  connections,  298. 
wave  propagation,  316. 


267 


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